“This has been the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see,” Greater Manchester Police chief constable Ian Hopkins said.

“Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives.”

He said a “fast-moving investigation” had established the attack was conducted by one man who died at the scene, although detectives are working to establish if he was “was acting alone or as part of a network”.

The attacker was carrying a device which he detonated.

The explosion occurred in the foyer of the arena at 10.30pm local time, as the concert was finishing. Witnesses reported hearing a huge bang, with videos posted on Twitter showing chaotic scenes as thousands of fans ran screaming from the packed venue.

Prime Minister Theresa May said police were treating the deadly incident as a terrorist attack and if confirmed it would rank as the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London’s transport system in July 2005.

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

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

Grande’s management expressed sorrow for the victims and families harmed in the “senseless attack”, with the singer herself tweeting:

https://twitter.com/ArianaGrande/status/866849021519966208

One concertgoer, 22-year-old Majid Khan, has told The Independent that she and her sister were exiting the venue after the show at around 10.40pm (local time) when a “huge bomb-like bang” went off that sent everyone into a panic as they tried to flee.

“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,” Khan said.

“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.”

Concert-goer Catherine Macfarlane has told Reuters she felt the blast in her chest before chaos erupted in the venue.

“We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming,” Macfarlane said.

“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.”

Desperate parents and friends are using social media to search for loved ones after the blast, with images of happy-looking teenagers posted next to pleas for help.

“Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn’t answering her phone, pls help me,” said one message posted alongside a picture of a blonde-haired girl with flowers in her hair.

Another Twitter user called Erin:P urged people to help him find his sister: “She’s wearing a pink sweatshirt and blue jeans. Her name is Whitney.”

A screen grab from a video posted on Twitter showing fans running from the Manchester Arena.

Gary Walker told the BBC he was injured by shrapnel as the explosion went off a few metres away while he and his wife waited for their daughters to emerge from the concert.

“We heard the last song go and then suddenly there was a massive flash and then a bang and smoke. I felt a pain in my foot and my leg,” Walker said.

“I turned around to my wife, who was standing at the side of me, and she said, ‘I need to lay down.’ She’s got a stomach wound and possibly a broken leg.

“I’ve got a bit of a hole in my foot where I’ve got a bit of shrapnel. I was surprised I got away so lightly.”

Walker said he saw metal nuts that he believed could have come from an explosive device.

Consular officials in London are trying to find out if any Australians have been injured or killed in the attack.

“In our high commission, they will be working hard to ascertain whether any Australians are impacted by this tragic event,” counter-terrorism co-ordinator Tony Sheehan told a Senate committee in Canberra this morning.

“Obviously, it’s a very serious incident and we will be paying very close attention throughout the day to the unfolding events.”

#Manchester. Please RT pic.twitter.com/WZWUb7ryxV

— Smartraveller (@Smartraveller) May 23, 2017

Prime Minister Malcolm Turnbull briefed government colleagues during a coalition joint parties meeting, noting the work of the High Commission.

Labor leader Bill Shorten tweeted that attack was every parent’s worst nightmare.

“Thinking of all those killed or hurt in the attack in Manchester. And the parents worried sick about their kids.”

Manchester Arena, the largest indoor arena in Europe, opened in 1995 and has a capacity for 21,000 people, according to its website. It is a popular concert and sporting venue.

-AAP