"Ariana Grande had just gone behind the curtain and the lights came up when there was this massive bang and a big cloud of smoke,” a concert-goer told The Guardian.
“I saw five people with blood all down them,” said one.
Robert Tempkin told the BBC: "Everyone was screaming and running, there were coats and people's phones on the floor. People just dropped everything. Some people were screaming they'd seen blood but other people were saying it was balloons busting or a speaker had been popped. There were lots of ambulances. I saw somebody being treated. I couldn't tell what had happened to him."
Witness Harry Johnson told a local radio station that when people heard the explosions, they ducked for cover as fast as possible.
"People were just looking around and no-one really knew what to do. Then there was just screaming and shouting," he said. "There was no space between anyone. Everyone was squished together trying to get out. It took around a minute to get out."
Karen Ford, who was at the concert with her daughter, told BBC it was chaos.
“The problem was a lot of children were there without parents. There was no one to calm them down so everybody was just screaming and crying and pushing,” she said. “There wasn’t any smoke, there wasn’t a flash, it literally was just a huge sound. My husband heard a second one outside. As we came out of the stairwell there wasn’t a steward to be seen ... it was just chaos.”
Witnesses recorded the hysteria and shared it on social media.
We'll update you as more information arrives.