Holly Jones, a BBC reporter who was on London Bridge at the time of the incident, said she had seen a man in handcuffs being taken away from the scene.

She said a white transit van had knocked several pedestrians over, estimating five or six casualties injured or seriously injured.

"A white van driver came speeding - probably about 50mph - veered of the road into the crowds of people who were walking along the pavement," she told BBC News.

"He swerved right round me and then hit about five or six people. He hit about two people in front of me and then three behind.

I'd say there are about four severely injured people. They all have paramedics assisting them at the moment."

Chris Wimpress, who works for the BBC, described the aftermath as "pandemonium".

He went to the bridge after being told of an unfolding incident.

"I was told there was something major happening," he said. "As I walked down Southwark Street heading east, I started to see dozens of people running away, clearly distraught. Mostly young people, many of them in tears.

"Men walking in an almost zombified state, looking shocked

"It appears a van came down from the city into the London Bridge area."

He added that witnesses had told him there was a stabbing in the Southwark Tavern, while others heard at least 12 gunshots in the area.

Two people, he said, had told him there were "men dressed in black"