A van has mounted the pavement on London Bridge before a stabbing attack, according to witnesses.
Just after 10pm police responded to reports of a vehicle colliding with pedestrians on London Bridge.
British Transport Police have said there are reports of “multiple casualties” on the bridge.
Shortly after that, officers said there had been reports of stabbings at Borough Market.
“Armed officers responded and shots have been fired,” Met Police said.
Police are also responding to an incident in the Vauxhall area, police said at 11.45pm.
Will Heaven was at London Bridge shortly after the incident and wrote on Twitter that he had seen casualties – one on the pavement and one on the edge of the road.
He told Sky News: “It was about 10.15pm I was in the back of an Uber cab driving over London Bridge…there was somebody down on the pavement with a small crowd around them clearly very concerned.
“It looked like someone had collapsed. We drove a bit further and noticed there was another person on the right hand side of the bridge who was also not on the pavement but in the road itself.
He added that “it did not look like an accident”, adding it was clear that police “thought they were responding to something far more serious than a road traffic accident”.
Reuters reported a witness saying she had seen people with “apparent knife wounds” on London Bridge.
Witness Nick Archer told Sky News that he had seen a man lying on the ground who “looked in a bad way” and managed to flag down one of the police vans “whizzing past”.
“Officers jumped out and started tending to the guy on the floor and about three of them also started running towards the London Bridge area.”
London Ambulance have said: “We’re responding to this incident and more information will follow when we have it.”
Police asked people to avoid the area, armed officers were seen at the scene and the bridge is closed in both directions. London Bridge, Bank, Vauxhall and Borough stations were closed, although Bank and Vauxhall have since reopened.
Tony Murphy, who is ex-military, lives near London Bridge and said: “About half an hour ago there was tremendous gunfire that started.
I thought at first it was fireworks, then I recognised it was significant gunfire.”
Sky News Home Affairs Editor Mark White said: “Certainly reports we’re getting suggest that this may be terror related but that is not being confirmed by police at this stage.
“However it is being declared a major incident.
“London Bridge would have been very busy with pedestrians at this time of night.”
A Downing Street spokesman said: “The Prime Minister is in contact with officials and is being regularly updated on the incident at London Bridge.”
Source: Sky News