Two members of the public have died after a stabbing attack at London Bridge, in which police also shot dead the suspect.
The Met Police has declared the attack a terrorist incident.
The suspect, who died at the scene, was wearing what is believed to have been a hoax explosive device, police said.
Videos on social media appear to show passers-by holding down a man. An officer arrives, seems to indicate to the group to move, and fires a shot.
A Whitehall source confirmed two members of the public died to the BBC but gave no further information.
Details are still emerging and Neil Basu, the head of UK counter-terrorism policing, said the force was keeping an open mind over the motive.
He said officers were called to a stabbing at a premises near the bridge just before 14:00.
BBC journalist John McManus, who was at the scene as it unfolded, said he had seen a struggle on the bridge, with several men restraining one man. Police then arrived quickly and a number of shots were fired at him, he said.
In footage shared on social media, the man could be seen being pinned down by members of the public. Another man in a suit could be seen running from him, having apparently retrieved a large knife.
Mr Basu told a press conference: "A male suspect was shot by specialist armed officers from City of London police and I can confirm that this suspect died at the scene."
He added that a "wide cordon remains in place" following the incident.
Mr Basu said: "Officers continue to carry out extensive searches in the area to ensure there is no further threat to the public."
There was no intelligence of anything about to happen, sources say, and police reacted to events as they unfolded.
At the start of this month, the UK threat level was lowered from severe to substantial - reflecting an assessment from the Joint Terrorism Analysis Centre that an attack was likely rather than highly likely because of a relative decline in activity.
The challenge for police and security services is that low-tech attacks - involving knives or vehicles - and often carried out by lone actors can be hard to spot in advance since they involve relatively little preparation and communication.
But in the wake of previous incidents in both the UK and other countries, police have been prepared for this kind of incident and seem to have been fast to intervene, taking few chances, although members of the public were also involved in restraining the individual.
The priority for police and MI5 now will be establishing the identity of the individual, whether he had any links or associations and whether there is any ongoing related threat.
The man was wearing what is believed to be a hoax explosive device, Mr Basu said.
BBC security correspondent Frank Gardner said attackers wear fake suicide vests for a couple of reasons: To add to public panic and in some cases to ensure police shoot them dead.
To some attackers, martyrdom is preferable to capture, trial and spending many years in prison, our correspondent added.
Mr Basu said the force would have extra police patrols across the capital.
Prime Minister Boris Johnson, who returned to Downing Street from his constituency, said the emergency services and members of the public who intervened "represent the very best of our country".
He said: "This country will never be cowed or divided or intimidated by this sort of attack and our values, our British values, will prevail."
The prime minister will convene a meeting of the government's emergency Cobra committee at 21:30.
London Mayor Sadiq Khan thanked the members of the public who showed "breathtaking heroism" in running towards danger - "not knowing what confronted them".
The Independent Office for Police Conduct said it has begun a mandatory investigation into the shooting.
London Bridge station and Tube has now reopened after it was earlier closed, Network Rail said.
But trains may continue to be delayed, cancelled or revised while services are reinstated, it added.
Both the Conservatives and Labour say they have suspended election campaigning for the night following the attack.
Witnesses have spoken to the BBC about what they saw.
Amanda Hunter, who was on a bus on London Bridge at the time, said: "All of a sudden [it] stopped and there was some commotion and I looked out the window and I just saw these three police officers going over to a man…
"It seemed like there was something in his hand, I'm not 100% sure. But then one of the police officers shot him."
Bus driver Mustafa Salih, 62, was travelling from Borough High Street towards London Bridge where he saw emergency vehicles and the police cordon.
He told BBC London: "A police officer came up to me and said 'turn off your engine, get off and run'.
"I looked up and I could see a crowd of people coming towards me.
"One woman was crying. I ran back down to Borough High Street. It was all very scary as we did not know what was happening."
London Bridge was the scene of another attack, on 3 June 2017, in which eight people were killed and many more injured.
This latest attack comes after the UK's terrorism threat level was downgraded on 4 November from "severe" to "substantial", meaning that attacks were thought to be "likely" rather than "highly likely".
The terror threat level is reviewed every six months by the Joint Terrorism Analysis Centre, which makes recommendations independent of government.
https://www.bbc.com/news/uk-50604781
