22 dead in terror attack at Ariana Grande concert in Manchester Arena

Armed police block a road near to the Manchester Arena in central Manchester, England, Tuesday, May 23, 2017. (AP Photo/Rui Vieira)

Greater Manchester police said at a press conference on Tuesday that they believe the attack was carried out by one man and he died at the arena. The attacker was carrying an improvised explosive device which he detonated.

"The priority is to establish whether he was acting alone or was part of a network," said Ian Hopkins, chief constable of Greater Manchester Police, adding that this was the "most horrific incident" that the city has ever faced.

In a statement at an earlier press conference, chief constable Hopkins said police are currently treating it as a "terrorist incident" until they have further information.

He said the injured are being treated at hospitals across the city and added that an emergency number has been set up for those worried about loved ones: 0161 866 9400.

Police are working with other officials to investigate the circumstances of the explosion and asked people to avoid the area around the arena while the investigation is ongoing, he said.

Reports of an explosion were first received at 10.33pm (local time), said police, adding that it happened at the end of the Ariana Grande concert, outside the venue as people were leaving.

Ambulances were seen rushing to the Manchester Arena venue and police added in an earlier statement that people should avoid the area.

"Emergency services responding to serious incident at Manchester Arena," Greater Manchester police said.

In another update, police said they carried out a "precautionary controlled explosion" in Cathedral gardens but confirmed that it was abandoned clothing. This followed reports that a suspected device was found.

Several media outlets reported that there had been two explosions from within the venue.

APPALLING TERRORIST ATTACK: THERESA MAY

British Prime Minister Theresa May condemned the "appalling terrorist attack".

"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack," May said in a statement.

She is set to hold a meeting of the country's top security committee at 9am (0800 GMT), and has suspended her campaign for the general election.

Mayor of London Sadiq Khan, sharing an earlier police statement in a twitter post, said the English capital stands with Manchester.

A witness who was at the venue in the northern English city of Manchester where US singer Ariana Grande had been performing said she felt a massive explosion as she was leaving the concert.

"We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming," Catherine Macfarlane told Reuters.

"It was a huge explosion - you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out of the area."

Witnesses told the BBC that people were being "crushed on the floor" as they scrambled for the exit at the arena.

Robert Tempkin, 22, from Middlesbrough, told the BBC: "Everyone was screaming and running, there were coats and people's phones on the floor. People just dropped everything.

"Some people were screaming they'd seen blood but other people were saying it was balloons busting or a speaker had been popped.

"There were lots of ambulances. I saw somebody being treated. I couldn't tell what had happened to him."

Suzy Mitchell, whose flat is opposite the venue said: "(I) just heard a huge bang from my bed, came out to the front of my apartment and everyone was running away in big crowds."

ARIANA GRANDE 'OKAY'

A spokesman for Ariana Grande said the singer "is okay".

Manchester Arena, the largest indoor arena in Europe, opened in 1995 and has a capacity for 21,000 people, according to its website. It is a popular concert and sporting venue.

Train services to and from Manchester Victoria Station - located under the Arena - had been cancelled.

Medics deploy at the scene of a reported explosion during a concert in Manchester, England on May 23, 2017. (Photo: PAUL ELLIS / AFP)

This is the deadliest attack in Britain since the London bombings in 2005. The country is on its second-highest alert level of "severe", meaning an attack by militants is considered highly likely.