"Panic", "chaos", "confusion" - these are just some of the words used by witnesses to describe what followed a blast at the end of an Ariana Grande concert in the northern English city of Manchester that killed at least 22 people and wounded dozens on Monday night.

"We just heard a bang," concert-goer Sadat Khan told Al Jazeera.

"A few seconds later there was a massive stampede of panic and shouting and screaming," Khan said.

He added that those near him, the majority of whom were children and teenagers, ran towards the exits, "jumping on chairs and trying to run as fast as they could".

Police on Tuesday said they are still gathering information about the explosion, which they are treating as a "terrorist incident".

Sam Ward, another witness, described a "huge" explosion.

"Imagine like a two-storey brick wall had separated itself from a building," Ward told Al Jazeera.

Video from inside the arena showed concert-goers screaming as they made their way toward exits amid a sea of pink balloons.

EXPLOSION AT MANCHESTER ARENA AND EVERYONE RAN OUT SO SCARY😭 pic.twitter.com/pJbUBoELtE — ♡♡ (@hannawwh) May 22, 2017

Catherine Macfarlane told the Reuters news agency the blast hit after the concert was over.

"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."

Police responded to reports of an explosion shortly after 10:35pm (21:35 GMT) at the arena, which has the capacity to hold 21,000 people, where the US singer had been performing to an audience that included many children.

Messages flooded social media as loved ones searched for news about family and friends who had attended the concert.

Near the arena, witnesses said the said they saw concert-goers fleeing in panic after hearing the explosion.

"We basically heard two loud bangs," Alan Brennan told Al Jazeera. "One was very quickly followed by the other."

He said he looked outside and could "see people running, screaming … there were lots of children screaming. It was horrific."

Paula Robinson, 48, was at the train station next to the arena with her husband when she felt the explosion and saw dozens of teenage girls screaming and running away from the arena.

Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to worried parents telling them to meet her there. She said her phone has not stopped ringing since her tweet.

"Parents were frantic running about trying to get to their children," she added.

A father who attended the concert with his young daughters spoke of disbelief.

"Why? What's the point? I mean we brought our children out for a day out thinking we'll have a really good concert. They'll remember the concert but probably for the wrong reasons. Innocent people."

Shortly after the explosion, Greater Manchester Police tweeted asking people to stay away.

"The incident took place outside the venue in a public space," police said. "Our thoughts and prayers go out to the victims."

The local ambulance service said on Twitter it had taken 59 casualties from the incident and treated "a number of walking wounded on scene".

British Prime Minister Theresa May condemned the incident, calling it 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," May said in a statement.

May will hold a meeting of the country's top security committee at 9am (08:00 GMT).

A "precautionary controlled explosion" was carried out near the venue on a package that was believed to be a suspect device, but turned out to be "abandoned clothing, not a suspicious item".

Manchester Arena, which opened in 1995, is the largest indoor arena in Europe, according to its website.

A spokesman for Ariana Grande's record label said the singer was "OK". The concert was part of Grande's The Dangerous Woman Tour.

Grande tweeted that she was "broken" after the incident.