With AAP.

Greater Manchester Police have confirmed 19 fatalities after an explosion occurred at a local concert venue.

Posting on Twitter, authorities confirmed that more than 50 others were also injured in the incident, which is currently being investigated by North West Counter Terrorism Unit.

British Transport Police say the explosion occurred in the foyer of the Manchester Arena at roughly 10.30pm local time, as a concert by US singer Ariana Grande was finishing.

Footage posted to Twitter showed fans, many of them young, screaming and running from the venue, which has the capacity to hold 21,000 people.

A witness cited that they saw “20 to 30 bodies” on the ground after the explosion occurred.

“There were bodies everywhere. I don’t know how we survived it, I really don’t know,” the witness said.

When the explosion went off after @arianagrande concert in @ManchesterArena hope everyone is safe. #Manchester

A post shared by Zach Bruce (@zachwhiteguybruce) on May 22, 2017 at 4:06pm PDT

UK Prime Minister Theresa May described the incident as “an appalling terrorist attack”.

“We are working to establish the full details of what is being treated by the police as an appalling terrorist attack,” she said in a statement.

“All our thoughts are with the victims and the families of those who have been affected.”

According to US news outlet ABC, preliminary investigations indicate a suicide bomber may be responsible for the blast.

However, local authorities are yet to comment on the source of the explosion.

Britain is on its second-highest alert level of "severe" meaning an attack by militants is considered highly likely.

Bomb disposal going into the arena. pic.twitter.com/FG3g4CkpoM

— Frances Perraudin (@fperraudin) May 22, 2017

A spokesperson for Ariana Grande earlier confirmed she is "okay" and safe following the evening's events.

The singer has since tweeted her sympathies, writing:

"Broken. from the bottom of my heart, i am so so sorry. i don't have words." [sic]

Her manager, Scooter Braun, added, "Tonight, our hearts are broken. Words cannot express our sorrow for the victims and families harmed in this senseless attack.

"We mourn the lives of children and loved ones taken by this cowardly act. We are thankful for the selfless service tonight of Manchester's first responders who rushed towards danger to help save lives."

I never ment to record this but this is me leaving ariana grande #DangerousWomantour #ManchesterArena hope everyone is safe and got out pic.twitter.com/TLu73UBl0x

— Rhianna burke (@RhiannaBurke) May 22, 2017

EXPLOSION AT MANCHESTER ARENA AND EVERYONE RAN OUT SO SCARY???? pic.twitter.com/pJbUBoELtE

— ♡♡ (@hannawwh) May 22, 2017

Concertgoers report hearing a "huge, bomb-like bang" go off around the time they were exiting the venue after the singer had finished performing.

"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," 22-year-old Majid Khan told Sky News.

"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."

Suzy Mitchell, 26, whose flat is opposite the venue, reported a huge bang rocking the neighbourhood.

She told the Press Association: "(I) just heard a huge bang from my bed, came out to the front of my apartments (we're on the top floor so have perfect view) and everyone was running away in big crowds.

"Currently lots of emergency services going to and from. But can't see anything substantial as of yet except fleeing people and lots of cars."

As heavily armed police continued to patrol the area, one woman posted on Facebook that over 50 children had been taken to the nearby Holiday Inn to wait for their parents.

"Please repost for any parents with children [at the concert]," Paula Robinson wrote on Facebook.

"We have taken as many kids as we can to Holiday Inn we will keep them safe and stay with them.

"We have got about 50 kids with us waiting to be picked up...they are safe we will look after them."

Liverpool City Region Metro Mayor, Steve Rotheram, tweeted that his two daughters were at the concert, but were safe.

He also added that waiting to hear for news of children is a "parent's worst nightmare".

My 2 daughters caught up in the Manchester explosion at the arena. They are thankfully safe, but I fear for others.

— Steve Rotheram (@Steve4LCRmayor) May 22, 2017

All my thoughts go out to those parents waiting to hear of the safety of their children. It's a parents worse nightmare. So, so sad.

— Steve Rotheram (@Steve4LCRmayor) May 22, 2017