Seven people have died and at least 20 have been taken to hospital after London was rocked by a terrorist attack last night.
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.
Shortly after 9:30am on Sunday morning, Metropolitan Police Commissioner Cressida Dick said the number of victims from the London attack has risen from six to seven.
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.
Witnesses described seeing a speeding vehicle veer into crowds on London Bridge.
A man called Gerard said he witnessed a series of stabbings by three men near Borough Market.
"They were stabbing everyone. They were running up and going 'This is for Allah'," he told BBC News.
He said he followed the attackers and threw items at them in a bid to stop them.
Alex Shellum, who was in The Mudlark pub, close to London Bridge, said he saw a bleeding woman in her early 20s stagger in.
Barman Alex Martinez was forced to take shelter in a bin when a knifeman stormed El Pastor, the Borough restaurant in which he worked.
He said: "I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening.
"The man started screaming and dropping everything on the floor, the plates, the cutlery, the bottle, everything, he dropped it."
He said he jumped down behind a wall to keep safe.
Mr Rowley added: "At this stage, we believe that six people have died in addition to the three attackers shot dead by police. And at least 20 casualties have been taken to six hospitals across London.
"We are reviewing and planning to strengthen our policing stance across London over the forthcoming days and there will be additional police and officers deployed across the capital.
"We are treating this as a terrorist incident and a full investigation is already under way, led by the Met's Counter Terrorism Command."
Holly Jones, a BBC reporter, was at London Bridge when she saw a van swerve into pedestrians.
"A white van driver came speeding - probably about 50mph - veered off the road into the crowds of people who were walking along the pavement," she told BBC News.
"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."
A taxi driver called Chris told LBC he saw men armed with foot-long knives after the van drove on to the pavement.
He told the station: "Then three men got out with long blades, 12 inches long and went randomly along Borough High Street stabbing people at random. I saw a young girl stabbed in the chest."