Greater Manchester Police said 19 people were killed and about 50 injured in a blast at Manchester Arena.
Witnesses said they heard two loud bangs, reported to be in the foyer, after a pop concert by American Ariana Grande.
Greater Manchester Police said there are a “number of confirmed fatalities and others injured”.
The cause of the explosion is unknown but the North West Counter Terrorism Unit is treating it as a possible terrorist incident.
According to the BBC, senior counter-terrorism officers are assembling in London and are liaising with the Home Office.
Early estimates put the number of fatalities in double figures, although these are not confirmed.
Manchester Victoria station, which is close to the concert venue, has been closed and all trains cancelled.
British Transport Police say explosion was in the foyer area of Manchester Arena.