Police say at least 22 people, some of them children, were killed and 59 others wounded in a suspected terrorist bombing outside an arena in the northern English city of Manchester.
Witnesses on May 22 reported hearing a "massive explosion" around 10:35 p.m. (2135 GMT/UTC) as people were leaving the Manchester Arena, where U.S. pop singer Ariana Grande was performing.
British police said 22 people were killed in the attack , which they believe was carried out by one man after the concert.
"We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins said.
"The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity."
"The priority is to establish whether he was acting alone or as part of a network," Hopkins said.
Reuters news agency reported that two U.S. officials had said initial signs pointed to a suicide bomber. U.S. Homeland Security officials said they were closely monitoring the situation.
The initial death toll had been put at 19.
In the aftermath of the incident, British political parties agreed to suspend Britain's election campaigning until further notice.
Jeremy Corbyn, the leader of the opposition Labour Party, said in a statement that he had spoken to Prime Minister Theresa May and had agreed that all national campaigning for the June 8 parliamentary vote would be suspended.
May earlier said that the explosion in the northern English city was being treated as a 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.
"I am horrified by the horrendous events in Manchester last night," Corbyn said.
"My thoughts are with families and friends of those who have died and been injured."
The North West Regional Ambulance Service said it had taken 59 casualties from the explosion to hospitals and treated a number of walking wounded.
Reuters news agency reported that two U.S. officials said initial signs pointed to a suicide bomber, although U.K. authorities have not confirmed the information.
Police urged people to avoid the area and said bomb-disposal teams were on the scene.
"Please avoid the area as first-responders work tirelessly at the scene," the police said.
Video shown on Sky News showed hundreds of people fleeing from the arena as police and emergency vehicles arrived. Witnesses reported seeing helicopters above the arena after the incident.
Other videos posted on Twitter showed fans screaming and running inside the venue.
Robert Tempkin, 22, of Middlesbrough told the BBC: "Everyone was screaming and running, there were coats and people's phones on the floor. People just dropped everything."
Service at the Manchester Victoria train station, which is located under the arena, had been canceled, officials said.
"Emergency services are at the scene and we are working to establish more information regarding the explosion and will provide further updates as soon as possible," the British Transport Police said.
A spokesman for Grande's record label said the singer was "OK."
If confirmed as terrorism, it would be the deadliest attack in Britain since four British Islamist extremists killed 52 people in suicide bombings on London's transport system in July 2005.
Manchester Arena, opened in 1995, is the largest indoor arena in Europe, with a capacity of 21,000 people.
The United Kingdom is on its second-highest alert level of "severe," meaning a terror attack is considered highly likely.
Sky News reported the government would hold an emergency security meeting at 9 a.m. (0800 GMT/UTC) on May 23 in London.
BBC said senior counterterrorism officers were assembling in London and liaising with the Home Office.