A number of people are believed to have been injured after a stabbing at London Bridge, police have said.
The Met said one man had been shot by police and it was treating the incident “as though it is terror related”.
The force earlier said it had detained a man. London Ambulance Service has declared a “major incident”.
The BBC’s John McManus, at the scene, said he had seen a group of men in a fight on the bridge. Police then arrived and shots were fired, he said.
British Transport Police said London Bridge station was currently closed and no trains would be stopping there.
Police have advised people near the scene to follow directions from officers on the ground.
The prime minister tweeted he was being updated on the incident and wanted to thank police and all emergency services for their response.
Home Secretary Priti Patel also tweeted that she was “very concerned” about the incident.
McManus told the BBC News channel: “Just a few minutes ago I was walking across London Bridge on the south bank to the north bank of the bridge.
“There appeared to be a fight going on on the other side of the bridge, with several men attacking one man.
“Police then quickly arrived, including armed police, and then a number of shots were fired at this man.”
A video has emerged showing police officers aiming guns at a white lorry that jack-knifed across London Bridge.
The footage shows several officers surrounding the vehicle before moving to the rear to check its container.
London buses can be seen either side of the lorry while three police cars are parked next to it on the bridge.
Another video filmed from a nearby bus showed the apparent suspect appearing to be shot as he lay on the ground.
Another man in a suit and jacket could be seen running from him, having apparently retrieved a large knife.
Witnesses have spoken to the BBC about what they saw.
Amanda Hunter, who was on a bus on London Bridge at the time, said: “All of a sudden [it] stopped and there was some commotion and I looked out the window and I just saw these three police officers going over to a man…
“It seemed like there was something in his hand, I’m not 100% sure. But then one of the police officers shot him.”
Noa Bodner, who is stuck in a restaurant on London Bridge, told BBC News channel: “There was a rush of people coming in and everybody basically dived under the tables.
“We were told to keep away from the windows, people that came from the outside were saying that shots were fired.”
She said the manager ran to lock the doors and staff told people to move away from the front of the restaurant.
She said the mood was “calm”, “some people seem a bit distressed, but they’re being looked after by friends or staff”.