There are reports of a number of casualties after an incident possibly involving a vehicle and a knife in the London Bridge area of the capital, British Transport Police has said.
Police say armed officers responded to reports of a van hitting pedestrians on the bridge in central London.
The Met Police said officers were also at Borough Market nearby where they were responding to reports stabbings.
A BBC reporter at the scene says police are searching for three suspects.
It said “multiple resources” are in attendance.
Downing Street said: “The prime minister is in contact with officials and is being regularly updated on the incident at London Bridge.”
Transport for London said the bridge has been closed in both directions due to a “major police incident”.
Bus routes were being diverted and the neighbouring Southwark Bridge has also been shut, it added.
TfL said there were further closures in Borough High Street, where armed police were later seen. On the north side of the river, Lower Thames Street was closed
BBC reporter Holly Jones, who was on the bridge at the time of the incident, said the van was driven by a man and was “probably travelling at about 50 miles an hour”.
“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,” Ms Jones told the BBC News Channel.
Five or six people were being treated for injuries after the vehicle mounted the pavement and hit them, she said.
“I’d say there are about four severely injured people. They all have paramedics assisting them at the moment.”
She said the van, which was travelling from the direction of central London, headed towards the south side of the river.
Ms Jones later reported seeing a man being arrested by police. She said he was handcuffed and had his shirt off.
She said a French woman was among the injured and who told her she did not know where the two people who had been with her were.
London Ambulance Service tweeted: “Multiple resources attending an incident at #LondonBridge, please avoid the area”.
Speaking to the Press Association, Will Orton described being in a pub in the area and seeing people coming running inside.
“We didn’t really know what was going on,” he said.
“We thought maybe there was a fight or something outside. And then there were almost hundreds of people coming inside.
“The bouncers did a really good job, they shut the doors and locked everyone in. There was panic – it seemed like it was literally outside the door. People were coming inside and saying they had witnessed people being stabbed.”
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: