Police sealed off London Bridge on Friday after a man reportedly started attacking people with a knife and was shot to death.

The BBC reported Met Police confirmed they were dealing with an “incident” on the bridge, which resulted in all traffic being removed. Sky TV reported as many as five people were injured and a second person was detained.

Scotland Yard said it was treating the incident as a terrorist attack.

Prime Minister Boris Johnson said anyone involved in the incident will be "hunted down and will be brought to justice." He said though the situation has been contained, the full extent is "not yet entirely clear."

Police said they would set up patrols this weekend across London, and U.S. President Donald Trump pledged support.

Witnesses circulated video footage on Twitter showing a man on the ground and another backing away from the scene with what appeared to be a knife in his hand. Armed officers arrived and surrounded the men before one of the officers pulled out a weapon and opened fire.

Neil Basu, the head of UK counter-terrorism policing, said in a statement the suspect was wearing a fake explosive device.

"We were in our van, and everybody started running," a witness told TalkRadio.

A woman in a nearby restaurant told the BBC  police ordered people to take cover.

"They went, 'Get under the table, shots have been fired. Gunshots'," the woman said.

Stuart Field was in a nearby coffee shop when the incident occurred.

People ran into the coffee shop. We saw three or four cars and a bus stopped in the northbound lane, almost like there had been an accident," he told the Financial Times. "People peeled out and were running. People were running for their lives. The store owners got us into the basement."

“There appeared to be a fight going on on the other side of the bridge, with several men attacking one man,” BBC reporter John McManus said. “Police then quickly arrived, including armed police and then a number of shots were fired at this man.”

He added: “Everybody has cleared the area, the entire bridge has been moved.”