Here is what we know so far about the attack, which was the deadliest in Britain since 2005.
Police said they were called at 2133 GMT to reports of an explosion at Manchester Arena at a concert by pop star Ariana Grande, who is popular with teenagers.
British Transport Police said the blast occurred at "within the foyer area of the stadium" and the venue said it was "outside the venue in a public space".
The arena foyer connects the 21,000-capacity auditorium with Victoria train and tram station, a major hub on the northern edge of the city centre.
Eyewitnesses described a "huge bomb-like bang" and scenes of panic as young fans rushed out and parents waiting outside looked for their children.
Who is behind it?
Police said the incident was "currently being treated as a terrorist incident," but no one has so far claimed the attack.
British Prime Minister Theresa May condemned it as "an appalling terrorist attack".
Greater Manchester Police's chief constable Ian Hopkins said local police in the city were working with national counter-terrorism and intelligence officials on the investigation.
Who are the victims?
Police said 19 people were killed and around 50 injured, who are being treated in six hospitals.
The local ambulance service said on Twitter it had taken 59 casualties from the incident and treated "a number of walking wounded on scene".
Elena Semino from Lancaster told the Guardian she was with her husband waiting for her daughter by the arena's ticket office when the explosion went off.
"There was this heat on my neck and when I looked up there were bodies everywhere," she said.
Gary Walker from Leeds told BBC Radio 5 Live he was hit by shrapnel in his foot and his wife sustained a stomach wound as they waited for their daughters to come out of the concert.
"We heard the last song go and then suddenly there was a massive flash and then a bang and smoke," he said.
Other attacks in the UK
The incident in Manchester is the second terror attack to hit England in less than two months.
On March 22, five people were killed and more than 50 injured when a man drove into pedestrians on Westminster Bridge in central London, before crashing into the fence surrounding parliament.
The attacker, 52-year-old Muslim convert Khalid Masood, was then shot dead by police at the scene after knifing a police officer to death.
Investigators described the lone-wolf attack as "Islamist related terrorism" but have not charged anyone in connection with the incident.
The deadliest bomb attack on British soil took place on July 7, 2005 when four British suicide bombers inspired by Al-Qaeda attack London's transport system, killing 52 and wounding 700.