Explosion in a subway in London

Source: Anadolu Agency / Getty

A live investigation is taking place following a Friday morning subway explosion in London that’s being treated as a terror attack.

According to Mark Rowley, Assistant Commissioner for London’s Metropolitan Police Service, the blast occurred at Parsons Green Tube Station shortly after 8 a.m. local time. ABC News reports at least 22 have been sent to the hospital, though no injuries are serious or life-threatening, according to The London Ambulance service.

Eyewitness Martin Adams tells ABC News he was riding in the subway at the time of the attack.

“I heard a loud bang and as I looked to my right, there was a flame, a fireball came through the carriage,” he recounts. “As the doors opened, people then began leaving the train straight away. I saw some flames coming from what I thought was a blue bag.”

Sally Faulding says she saw people with burns as she watched panic ensue on the subway platform.

“I also saw people injured obviously from having been stampeded on the platform because we were all running,” she said. “People were falling over.”

Police say the cause of the blast was an “improvised device” that failed to fully explode. So far, no arrests have been made.

London Mayor Sadiq Khan said in a statement: “Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life. As London has proven again and again, we will never be intimidated or defeated by terrorism.”

President Trump responded to the news via Twitter at 6:42 a.m. EST, calling those responsible for the attack “sick and demented people.”

On Friday afternoon, the UK’s terrorism threat level was raised to “critical,” with another attack expected to happen soon.

This story is developing.