Police said there was a number of confirmed fatalities, and many others were injured after an explosion in the foyer of the arena.

Loud bangs were heard about 10.35pm following the concert by US pop singer Ariana Grande. A representative for Grande told Variety she is "okay".

Police are treating it as a terrorist incident. A bomb disposal team is reportedly on site. 

"So far 19 people have been confirmed dead, with around 50 others injured," Greater Manchester Police said in a statement. 

"This is currently being treated as a terrorist incident until police know otherwise. 

All train services to Victoria Station, underneath the arena, have been cancelled and the station evacuated. 

Much of the concert crowd, estimated at 21,000, were young people and children - unaccompanied or with their parents.

Outside the venue after the blast there were desperate scenes as parents attempted to find children who had attended the concert.

A spokesperson for the Department of Foreign Affairs and Trade said it was not known if any Australians were involved. 

"There have been reports of an explosion at an Ariana Grande concert in Manchester UK on the evening of 22 May (local time). The Australian High Commission in London is making urgent enquiries to determine whether any Australians have been affected by the incident."

Concertgoers flee after explosion

Andy Holey, whose wife and daughter were at the concert, told BBC News that the explosion was near to a ticket office, and he was blown off his feet when it happened, and was unconscious.

He said he saw several bodies around him when he came to.

A witness who said the explosion was so big "you could feel it in your chest".

"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."

Karen Ford told the BBC the explosion went off after the lights went on at the end of the concert.

"All of a sudden huge sound... an explosion went off," she said.

"Everyone just stopped and turned round then somebody shouted 'it's a bomb' and everyone started running.

"People were pushing each other aside in panic, she said, and there were "a lot of children without parents with no-one to calm them down.

"There was screaming and crying and pushing. We were being crushed as we exited," she said.

Sasina Akhtar told the Manchester Evening News that she saw "young girls with blood on them".

"She [Grande] did her last song, we were in the lower tier and there was an explosion behind us at the back of the arena," she said.

"We saw young girls with blood on them, everyone was screaming and people were running. There was lots of smoke."

Steve Rotheram, Mayor of the Liverpool City Region, tweeted that his two daughters were "caught up in the Manchester explosion" but were safe.

One eyewitness told BBC he saw a man covered in blood.

"There was also a woman outside who looked like she had been hurt, she was on the floor," Josh Elliott said.

He added: "I think what the injuries have come from is people trying to get out as quickly as possible."

He said that "massive" balloons had come down into the arena during one of Grande's songs.

"My initial reaction was for me and my friends to get on the floor," he said. "I thought it was probably just a balloon, but at the same time in situations like this you just don't know what it is and it's better to be safe than sorry."

One fan at the concert told London's Telegraph how the explosion hit the venue as fans were leaving the building.

Majid Khan, 22, said: "I and my sister, along with a lot of others were seeing Ariana Grande perform at Manchester Arena, and we were all exiting the venue when around 10.40-10.45pm-ish a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.

"It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit Trinity Way and that was blocked so everyone was just running to any exit they could find as quickly as they could.

"Everyone was in a huge state of panic, calling each other as some had gone to the toilet whilst this had gone off, so it was just extremely disturbing for everyone there."

Ariana Grande, a 23-year-old singer with a big voice who started her career as a star on a Nickelodeon TV series, is on an international tour supporting her 2016 album, Dangerous Woman.

The tour was scheduled to continue on Thursday at the O2 Arena in London. Two additional acts, Victoria Monet and Bia, performed as openers.

Fairfax Media