A British Transport Police spokesman said: 'We are aware of an incident at Manchester Arena. We have officers at the scene and will provide further updates as soon as possible.'
In a statement, Manchester Arena said: 'We can confirm there was an incident as people were leaving the Ariana Grande show last night. The incident took place outside the venue in a public space. Our thoughts and prayers go out to the victims.'
US pop singer Grande was performing her Dangerous Woman European tour at Manchester Arena when the incident happened.
Later this week, she is scheduled to perform in London, followed by concerts in Belgium, Poland and Germany. A spokesman for the singer said she was unharmed but inconsolable.
Manchester Arena, the largest indoor arena in Europe, opened in 1995 and is a popular concert and sporting venue.
Britain is on its second-highest alert level of 'severe' meaning an attack by militants is considered highly likely.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
[h=3]GLOBAL HISTORY OF TERROR ATTACKS AT ENTERTAINMENT VENUES AND NIGHTCLUBS[/h]November 13, 2015 - Bataclan Theatre: The Paris terror attacks included a bloody mass shooting at the theatre during an Eagles of Death Metal rock gig. There were 89 deaths in the theatre, and a total of 130 across the French capital on the night of Friday the 13th. A total of 368 people were injured across the city. Seven of the Islamic State attackers were killed.
November 13, 2015 - La Carillon bar: The Paris terror attackers also targeted a bar, where 20 people were killed.
December 4, 2015 - Cairo restaurant: Firebombs were thrown into the El Sayad restaurant in Egypt's Cairo, killing 16 people and injuring three others. Victims died from smoke inhalation or burns, after struggling to escape from the basement venue which had only one exit.
June 12, 2016 - Orlando nightclub: Omar Mateen, 29, slaughtered 49 people and injured 53 others at a gay nightclub in Orlando, Florida, last year. Mateen was shot dead after a three-hour standoff with police at the Pulse nightclub. The attack was the deadliest mass shooting by a lone gunman and the deadliest violence against LGBTQI people in the history of the United States. The shooter swore allegiance to the Islamic State in a 911 call to police, but an investigation found Mateen had no links to the terror group.
June 28, 2016 - Kuala Lumpur nightclub: A grenade was thrown into the crowd at Movida nightclub in Kuala Lumpur, Malaysia, in June last year. Eight people were injured. In March this year, a high court judge sentenced two people to 25 years' jail for the attack, as well as an additional 10 years for supporting the Islamic State.
New Years' Eve 2016-17 - Istanbul nightclub: Partygoers were welcoming in 2017 when a shooter sprayed bullets into Reina nightclub and killed 39 people on New Years' Eve in Turkey's Istanbul. The alleged gunman, Abdulkadir Masharipov, who was born in Uzbekistan, was arrested after two weeks on the run.
January 1, 2016 - Tel Aviv bar: Two people were killed at Israel's Simta bar on New Years' Day last year while up to seven others were wounded. Witnesses claimed the gunman was smiling calmly while shooting people down at random with a machine gun. The gunman then killed a taxi driver as he fled the scene. Nasha'at Melhem, 31, was named as the killer and he was shot dead in a shoot-out with police a week later.