Armed police gather at Manchester Arena after reports of an explosion at the venue during an Ariana Grande gig in Manchester, England Monday, May 22, 2017. Several people have died following reports of an explosion Monday night at an Ariana Grande concert in northern England, police said. A representative said the singer was not injured. (Peter Byrne/PA via AP)

At least 22 people were killed and 59 wounded in an explosion at the end of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday, in what two U.S. officials said was a suspected suicide bombing.

Prime Minister Theresa May said the incident was being treated as a terrorist attack.

Police responded to reports of an explosion shortly after 10:33 pm (2133 GMT) at Manchester Arena, which has the capacity to hold 21,000 people, where the U.S. singer had been performing to an audience that included many children.

A witness who attended the concert said she felt a huge blast as she was leaving the arena, followed by screaming and a rush by thousands of people trying to escape the building.

A video posted on Twitter showed fans, many of them young, screaming and running from the venue. Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.

“We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming,” concert-goer Catherine Macfarlane told Reuters.

Police and other emergency services evacuate survivors

“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.”

Ariana Grande, 23, later said on Twitter: “broken. from the bottom of my heart, i am so so sorry. i don’t have words.”

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

May will hold a crisis response meeting on Tuesday.

Chinese President Xi Jinping sent his condolences over the blast to Britain’s Queen Elizabeth, Chinese state media reported.

Islamic State supporters took to social media to celebrate the blast and some encouraged similar attacks elsewhere.

President Kenyatta condemned the attack and assured UK Prime Minister Theresa May that Kenya would offer any help possible and will stand with Britain in the fight against extremism.

“Kenya knows what it is like to suffer a tragedy such as this, so our hearts go out to the UK, and especially to the families which have woken to devastating news this morning,” the President said adding that Kenya shares a long history with the UK.

Here is a video of crowds fleeing the scene: