A number of people are believed to have been injured after a stabbing at London Bridge, police have said.
The Met Police said they were called to a stabbing at a premises near the bridge just before 14:00.
The force 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.”