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May 23, 2017, 12:47 AM UTC / Updated May 23, 2017, 7:44 AM UTC / Source : Reuters

Thousands fled in a state of panic after a suicide blast after an Ariana Grande concert that killed at least 22 people on Monday night.

The singer had just performed her encore when a loud bang was heard.

Concertgoer Zac Haniff told NBC News that he initially believed someone had dropped a microphone backstage.

“We then heard people crying and screaming and we were really confused,” he added.

Ivo Delgado was in line to leave when he heard the bang. Then he saw smoke, people lying on the floor and someone with a bloody face.

“People started screaming and going to the other side of the arena," he told NBC News.

Video from inside the venue showed concertgoers shouting and leaping over a stairwell handrail. Outside, people could be seen racing down long flights of stairs.

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Authorities said the blast occurred around 10:33 p.m. local time (5:33 p.m. ET) outside the arena.

"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concertgoer Majid Khan, 22, told the Associated Press. "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."

Attendee Catherine Macfarlane described it as a "huge explosion."

"You could feel it in your chest," she told Reuters. "It was chaotic. Everybody was running and screaming and just trying to get out."

Desperate parents and friends used social media to search for loved ones who attended Monday's concert while the wounded were being treated at six hospitals across Manchester.

"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 girl with flowers in her hair.

Paula Robinson, 48, said she 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 arena.

"We ran out," Robinson told Reuters. "It was literally seconds after the explosion. I got the teens to run with me."

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 had not stopped ringing since her tweet.

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