Seven people have died and at least 40 have been taken to hospital after the capital was rocked by the country's second terrorist attack in less than two weeks.
Officers responded to reports of a vehicle ploughing into pedestrians around London Bridge and stabbings at Borough Market shortly after 10pm.
Three attackers, wearing hoax explosive vests, were shot and killed by police.
Witnesses have described seeing a speeding vehicle veer into crowds on London Bridge before multiple attackers stabbed scores of people in Borough Market.
An on-duty British Transport Police officer received "serious injuries" including head, face and leg wounds, as he responded to calls for help from the public, Scotland Yard said.
Assistant Commissioner Mark Rowley said: "We believe three people were involved but we still have some more inquiries to be 100% confident on that."
Police said they had received reports of a vehicle striking pedestrians on London Bridge at around 10pm, which then continued on to Borough Market.
The suspects then left the vehicle and a number of people in the area were stabbed, Scotland Yard said.
Here is a recap of how the terror unfolded:
- Police were called at around 10.08pm to reports of a vehicle striking pedestrians on London Bridge;
- The white van then continued to drive to Borough Market;
- Three men then left the vehicle and began to stab people, including a British Transport Police Officer;
- The suspects were confronted and shot by police at Borough Market within eight minutes of the first call being made.
Witnesses reported seeing a vehicle travelling at about 50mph strike people walking along the pavement.
Holly Jones, a BBC reporter, who was at London Bridge, said: "A white van driver came speeding - probably about 50mph - veered off the road into the crowds of people who were walking along the pavement.
"He swerved right round me and then hit about five or six people. He hit about two people in front of me and then three behind."
Witness Eric described the terror attack as a "rampage".
He told BBC Radio 5 Live he saw three people get out the van and initially believed they were going to help those who had been injured.
He said: "The three people literally started kicking them, punching them, and took out knives. It was a rampage really."
The three men ran off towards the nearby bars and restaurants and there was a shout of "this is for Allah", he told the station.
Another witness called Gerard said the suspects stabbed a series of people near Borough Market and shouted, "This is for Allah".
Liam Connell was in Katzenjammers Bierhall in Southwark Street, where customers were told to evacuate by staff before armed police told them to get on the ground.
He told the Press Association: "Then someone came in off the street and he had a white cuff around his neck.
"(The man) said he had been stabbed in the neck and I was holding his bandage and wound while he was talking to the police."
Guy's and St Thomas' NHS Foundation hospital said it was "on lockdown" to keep patients, relatives and staff safe, while a picture on social media showed armed police outside the Andaz Hotel in Liverpool Street, where the walking wounded were reportedly taken.
London Ambulance Service Assistant Director of Operations Peter Rhodes said: "We took 48 patients to five hospitals across London and treated a number of others at the scene for minor injuries."
Assistant Commissioner Mark Rowley of the Metropolitan Police said his officers were treating the incident as terrorism and additional police would be deployed in the capital.
Police have opened a casualty bureau line for people concerned about loved ones on 0800 0961 233.
The attack has received widespread condemnation from political leaders, with London mayor Sadiq Khan calling it "deliberate and cowardly".
He said: "This was a deliberate and cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night.
"I condemn it in the strongest possible terms. There is no justification whatsoever for such barbaric acts."
Prime Minister Theresa May said: "Our thoughts are with those who are caught up in these dreadful events."
The Government's Cobra emergency committee will meet on Sunday, Downing Street said.