At least one person has been killed and multiple people injured in a suspected terror attack around London Bridge.
A person is believed to have started attacking people with a knife and was then, according to witnesses, shot by police on the north side of the bridge.
Footage circulating on social media showed members of the public initially detaining the man and taking possession of the knife.
One person, who is not the suspect, has been killed, and "a number of people have been stabbed," police said.
Met Police will be making a statement at 4pm.
A Met spokesman said: "Police were called at 1.58pm to a stabbing at premises near to London Bridge. Emergency services attended, including officers from the Met and City of London Police.
"A man has been detained by police. We believe a number of people have been injured."
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Just after 3.20pm, Met Police added: "At this stage, the circumstances relating to the incident at London Bridge remain unclear.
"However, as a precaution, we are currently responding to this incident as thought it is terror-related.
"One man has been shot by police."
Video from the bridge shows a group of up to 10 pedestrians tackling a person, with one pedestrian wearing a suit and tie grabbing a large knife, before police tell them to get off the person.
Police then pull a final member of the public off the man and two shots are then heard.
Another video taken from a different angle shows one armed police officer at the beginning of the northside of the bridge crouched behind a police car aiming a gun down the bridge.
The video then moves to an unresponsive person on the pavement on the east side of the bridge.
Another video showed three officers pulling a man wearing jeans off the person on the ground before they step back.
Craig Heathcote, who was walking across the bridge at the time, told Sky News: "I reached the north side and someone came running towards me saying 'get out of the way, someone's got a knife'.
"I could then see on the other side of the bridge a big scuffle of people trying to get someone to the ground.
"I just immediately called 999. It was quite hard because cars were passing in the way of what was happening.
"A police car appeared from the bridge and two police officers got out and started taking over.
"There was lots of shouting and them trying to restrain him or get people out of the way before they fired the guns."
Pedestrians on London Bridge, one of the main Thames crossings into the City, were seen running back towards the southside of the bridge, while drivers were seen abandoning their cars and running back.
Borough Market, which is on the south side of the bridge, has been evacuated.