At least one person was killed and five 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.
Footage circulating on social media showed members of the public initially detaining the man and taking possession of the knife.
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.”
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 back.
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.
Offices around the bridge are on lockdown and London Bridge station is closed, while trains and the Tube are not stopping, British Transport Police said.
