- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/151226130034-vienna-police-terror-attacks-europe-nr-00000000.jpg?q=x_5,y_207,h_2662,w_4731,c_crop/h_540,w_960" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/151226130034-vienna-police-terror-attacks-europe-nr-00000000.jpg?q=x_5,y_207,h_2662,w_4731,c_crop/h_540,w_960" } }" data-vr-video="false" data-show-html=" Newsroom " data-byline-html="
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Updated 8:20 PM EST, Sat December 26, 2015
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Police stand outside the Palais Coburg palace where are taking place the so-called EU 5+1 talks with Iran in Vienna, Austria on October 16, 2014. Six world powers and Iran tried to get troubled nuclear talks back on track after a day of discussions led by US Secretary of State John Kerry yielded no breakthrough. AFP PHOTO / SAMUEL KUBANI (Photo credit should read SAMUEL KUBANI/AFP/Getty Images)
Several cities warned about possible terror attack
01:37 - Source: CNN
Threats in the UK 16 videos
- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/151226130034-vienna-police-terror-attacks-europe-nr-00000000.jpg?q=x_5,y_207,h_2662,w_4731,c_crop/h_540,w_960" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/151226130034-vienna-police-terror-attacks-europe-nr-00000000.jpg?q=x_5,y_207,h_2662,w_4731,c_crop/h_540,w_960" } }" data-vr-video="false" data-show-html=" Newsroom " data-byline-html="
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2015-12-26T18:06:05Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2015/12/26/vienna-police-terror-attacks-europe-nr.cnn" data-branding-key="" data-video-slug="Vienna police terror attacks europe nr" data-first-publish-slug="Vienna police terror attacks europe nr" data-video-tags="austria,christmas,europe,government and public administration,government bodies and offices,holidays and observances,intelligence services,international relations and national security,national security,terrorism,terrorism and counter-terrorism,terrorist attacks,unrest, conflicts and war,vienna,western europe" data-details="">
Police stand outside the Palais Coburg palace where are taking place the so-called EU 5+1 talks with Iran in Vienna, Austria on October 16, 2014. Six world powers and Iran tried to get troubled nuclear talks back on track after a day of discussions led by US Secretary of State John Kerry yielded no breakthrough. AFP PHOTO / SAMUEL KUBANI (Photo credit should read SAMUEL KUBANI/AFP/Getty Images)
Several cities warned about possible terror attack
01:37
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Updated 8:20 PM EST, Sat December 26, 2015
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Darlene Horton
American killed in London knife attack
00:51
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2016-08-04T20:03:53Z" data-video-section="tv" data-canonical-url="https://www.cnn.com/videos/tv/2016/08/04/london-knife-attack-phil-black-pkg.cnn" data-branding-key="" data-video-slug="london knife attack phil black pkg" data-first-publish-slug="london knife attack phil black pkg" data-video-tags="crime, law enforcement and corrections,crimes against persons,criminal offenses,england,europe,homicide,international relations and national security,investigations,london,national security,northern europe,stabbings,terrorism,terrorism and counter-terrorism,terrorist attacks,united kingdom,unrest, conflicts and war" data-details="">
A police officer guards the scene of a knife attack on August 4, 2016 in London.
Terrorism ruled out in deadly London knife attack
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Updated 8:20 PM EST, Sat December 26, 2015
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Mark Rowley statement
Police: 19-year-old arrested in London knife attack
02:39
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2016-08-04T03:33:44Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2016/08/04/london-knife-attack-black-lklv.cnn" data-branding-key="" data-video-slug="london knife attack black lklv" data-first-publish-slug="london knife attack black lklv" data-video-tags="crime, law enforcement and corrections,crimes against persons,criminal offenses,england,europe,homicide,london,northern europe,stabbings,united kingdom,diseases and disorders,health and medical,wounds and injuries" data-details="">
london stabbing 2
1 dead, 5 injured in London knife attack
02:01
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2016-08-04T01:38:55Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2016/08/04/london-knife-attack.cnn" data-branding-key="" data-video-slug="london knife attack" data-first-publish-slug="london knife attack" data-video-tags="crime, law enforcement and corrections,crimes against persons,criminal offenses,diseases and disorders,england,europe,health and medical,london,northern europe,stabbings,united kingdom,wounds and injuries" data-details="">
knife attack london map
Suspect detained after London knife attack
01:34
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2015-12-07T08:37:25Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2015/12/07/london-tube-stabbing-suspect-pleitgen-lklv.cnn" data-branding-key="" data-video-slug="london tube stabbing suspect pleitgen lklv" data-first-publish-slug="london tube stabbing suspect pleitgen lklv" data-video-tags="business and industry sectors,crime, law enforcement and corrections,crimes against persons,criminal law,criminal offenses,england,europe,international relations and national security,law and legal system,london,national security,northern europe,public transportation,stabbings,subway and commuter rail,terrorism,terrorism and counter-terrorism,terrorist attacks,transportation and warehousing,united kingdom,unrest, conflicts and war" data-details="">
A police car is seen parked outside Leytonstone station in north London on December 6, 2015. Police were called to reports of people being attacked at Leytonstone around 19:00 GMT on December 5. The police have said that they are considering a knife attack the previous evening as a " terrorist incident" after reports that the man reportedly shouted "this is for Syria". The man was arrested after being Tasered by police. One man suffered serious knife injuries while two others received minor injuries. / AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)
London stabbing suspect charged with attempted murder
01:09
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2015-12-06T12:36:53Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2015/12/06/london-stabbings-underground-tube-station-sot-black.cnn" data-branding-key="" data-video-slug="london stabbings underground tube station sot black" data-first-publish-slug="london stabbings underground tube station sot black" data-video-tags="" data-details="">
London attack being investigated as terrorist incident
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Updated 8:20 PM EST, Sat December 26, 2015
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uk terror drill
London police focus on new anti-terror tactic
02:01
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Updated 8:20 PM EST, Sat December 26, 2015
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Police: London tube station stabbings a terror attack
00:10
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- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/150707153711-london-7-7-memorial-s050268530.jpg?q=x_279,y_48,h_1628,w_2893,c_crop/h_540,w_960" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/150707153711-london-7-7-memorial-s050268530.jpg?q=x_279,y_48,h_1628,w_2893,c_crop/h_540,w_960" } }" data-vr-video="false" data-show-html="" data-byline-html="
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Updated 8:20 PM EST, Sat December 26, 2015
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LONDON, ENGLAND - JULY 07: London Mayor Boris Johnson (L) British Prime Minister David Cameron place wreathes at the July 7 memorial in Hyde Park on July 7, 2015 in London, England. Today is the tenth anniversary of the 7/7 bombings, when four suicide bombers struck transport system in central London on Thursday 7, July 2005, killing 52 people and injuring more than 770 in simultaneous attacks. (Photo by Steve Parsons- WPA Pool/Getty Images)
UK honors victims of 7/7 bombings on 10th anniversary
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Updated 8:20 PM EST, Sat December 26, 2015
" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2015-07-06T19:27:19Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2015/07/06/7-7-remembering-the-victims-of-london-terror-bombings.cnn" data-branding-key="" data-video-slug="7 7 remembering the victims of london terror bombings" data-first-publish-slug="7 7 remembering the victims of london terror bombings" data-video-tags="accidents, disasters and safety,bombings,disaster relief,england,europe,international relations and national security,london,national security,northern europe,rescue operations,terrorism,terrorism and counter-terrorism,terrorist attacks,united kingdom,unrest, conflicts and war" data-details="">
7 7 remembering the victims of london terror bombings_00014811.jpg
Memories of the 7/7 London terror attacks
02:41
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Updated 8:20 PM EST, Sat December 26, 2015
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Police apprehend 3 teens reportedly en route to Syria
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Updated 8:20 PM EST, Sat December 26, 2015
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Countering London's terror threats
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Updated 8:20 PM EST, Sat December 26, 2015
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Updated 8:20 PM EST, Sat December 26, 2015
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London terror suspect ID'd by neighbors
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Story highlights

Vienna police warn European cities of possible terror threats

The possible attacks involve explosives or guns set for some time during the holidays

Vienna and other police in Europe have heightened security

CNN  — 

An unnamed “friendly” intelligence service has warned several European cities of possible terror attacks, according to a statement released Saturday by police in Vienna, Austria.

The attacks would involve explosives or guns and occur sometime between Christmas and New Year’s Eve, according to the statement, which did not name the cities that have been warned.

The warning did include the names of several possible attackers the Vienna police have investigated without finding “concrete further results.”

“Overall, this is a lead, which stipulates a higher than general abstract state of danger,” the Vienna police said.

00-07 Laurie Segall New York :08-:15 CNN (no need to font) :37-:41 Reuters :51-1:07 Getty Images 1:10-1;30-SOT Moxie Marlinspike Encryption Specialist 1:40-1:45 CNN 1:45-1:50 WhatsApp 1:54-end Laurie Segall New York
00-07 Laurie Segall New York :08-:15 CNN (no need to font) :37-:41 Reuters :51-1:07 Getty Images 1:10-1;30-SOT Moxie Marlinspike Encryption Specialist 1:40-1:45 CNN 1:45-1:50 WhatsApp 1:54-end Laurie Segall New York
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In response to the terror threat warning, Vienna and other police in Europe have heightened the security alert by increasing police observation and surveillance at public venues, especially at key events and high-traffic areas.

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Among the precautions, police will initiate more thorough security checks, ensure quick readiness in case of an emergency, and increase vigilance in terms of empty suitcases and bags, Vienna police said in the statement.

Sensitive sites

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French National Police refused to comment on the warnings when contacted by CNN, but did say that more than 48,000 police officers are dedicated to security at sensitive sites during the school holidays from December 19 through January 4, and France plans to recruit 2,000 new police officers next month.

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A Belgian prosecutor who serves as a government spokesman on terrorism issues also refused to comment when contacted by CNN.

CNN received no immediate responses to requests for comment from officials of European cities.

CNN’s Margot Haddad and Laura Goehler contributed to this report.