A man has been shot and a number of people injured in stabbings at London Bridge, Scotland Yard said, adding that the incident is being treated as terror-related as a precaution.
The scene, in an extremely busy area in the heart of the British capital, remains cordoned off with heavy police and emergency services presence.
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 though it is terror-related. One man has been shot by police, a Metropolitan Police statement said.
London Ambulance Service had earlier declared it a major incident.
An earlier police statement had described the exchange as a stabbing incident at a premises in London Bridge during which a man was detained.
Images appearing on social media appear to show an exchange with the potential suspect being detained by police officers.
There are also some videos showing people running across the bridge in panic.
The transport on London Bridge remains at a standstill and offices and buildings in the vicinity are under lockdown as the incident unfolded.
British Transport Police said London Bridge train station was closed and no trains would be stopping there.
British Prime Minister Boris Johnson broke from his General Election campaign engagements to rush back to Downing Street.
"I'm being kept updated on the incident at London Bridge and want to thank the police and all emergency services for their immediate response," he said.
UK Home Secretary Priti Patel issued a Twitter statement to express her concern as reports on the incident poured in.
My thoughts are with all affected. I am grateful for the rapid response of our police and I urge everyone to follow their advice, she said.
London Mayor Sadiq Khan described it as a horrendous incident, in which a number of people were injured.
We must and we will stay resolute in our determination to stand strong and united in the face of terror. Those who seek to attack us and divided us will not succeed, he said.
London Bridge was one of the areas targeted by an ISIS-claimed terrorist attack in June 2017, when 11 people died as terrorists went on a stabbing spree after ramming a van into pedestrians.
The UK had just earlier this month downgraded its terror threat level from "severe" to "substantial", which means a terrorist threat is likely rather than highly likely in the country.