A number of people are believed to have been injured in an incident at London Bridge, police have said.
The Met said they were called to a stabbing at a premises near the bridge just before 14:00.
The force said they 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.
Police have advised people near the scene to follow directions from officers on the ground.
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.”
MAST: Sources on social media have posted an image which appears to show a lorry halfway across both carriages of the road, as well as around seven police cars. Video from the scene shows armed officers engaged with one suspect on the ground. The incident is ongoing.