A police boat on the River Thames as abandoned buses are parked on London Bridge in London. Picture: AP Photo/Frank Augstein

London - British police shot a man on Friday after a stabbing in the London Bridge area in the centre of the city.

The following is a Reuters summary of what we know and do not know about the incident.

* Death toll: British police said officers had shot a man in an incident at London Bridge on Friday. Sky News reported that another person had been killed.

The police said that the circumstances of the incident were unclear, but as a precaution it was being treated as though it were terror-related.

* Police said they were called at 1:58 p.m. to a stabbing on a premises near London Bridge.

* A 14-second video clip on Twitter filmed from a high vantage point showed what appeared to be three police officers backing away from a man lying on the pavement.

Videos and photographs also showed several police cars and buses on the bridge and a truck straddling several lanes. Reuters could not independently verify the footage.

It is so far unclear who was shot.

* London's ambulance service declared what it called a "major incident" in the area and London Bridge station was closed.

* British Prime Minister Boris Johnson said he was returning to his office and that he was being kept informed of developments.

"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," Johnson said.

* In June 2017 three attackers rammed a van into pedestrians at London Bridge, stabbing people in nearby bars. Eight people were killed and at least 48 injured. Islamic State said its militants were responsible.