Explosion kills 22 people at Ariana Grande concert in U.K.

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(SkyNews HD)

United Kingdom police are treating an explosion outside an Ariana Grande concert at Manchester Arena as a "terrorist incident". At least 22 people, including the bomber, are dead and more than 50 wounded, police said.

Witnesses told news outlets that they heard a "loud bang" and then people started screaming and running out of the arena.

Bomb disposal teams responded to the arena. Authorities advised the public to avoid the area around the arena while emergency responders help victims and conduct an investigation.

Hours after the initial incident, Manchester police carried out a "controlled explosion," indicating that police found something suspicious. But later police said the item was "abandoned clothing, not a suspicious item."

A record label representative said that Grande is "OK" but did not provide further details. She is in the midst of a world tour. Grande had already finished her set when chaos erupted.

"There's a large police presence here, police helicopter overhead as well. Quite a large cordon now and they are asking people to stay away," said Sky News producer Steph Oliver, who is outside the arena. "One lady told me she heard a loud explosion, they were asked to leave the arena quite quickly, they saw a number of people treated by ambulances."

The Manchester Arena tweeted that the "incident took place outside the venue in a public area."

The NYPD is "closely monitoring events in Manchester, England, to determine any possible implications for #NYC," Commissioner James O'Neill said in a tweet. Police officers armed with assault rifles were posted around Madison Square Garden in Manhattan.

Later, the NYPD issues a statement saying that in touch with English authorities.

"Out of an abundance of caution, we have moved additional heavy weapons teams to high profile locations around the city," Assistant Commissioner J. Peter Donald said in the statement. "In addition to our heavy weapons teams, New Yorkers will also see vapor wake explosive detection dogs, highly trained counterterrorism officers, and random bag checks at transit locations, among other things."

With the Associated Press and Sky News.