Police are responding to reports that a van has hit a number of pedestrians on London Bridge in central London.
Witnesses have said that armed officers are understood to be at the scene after a white van mounted the pavement before driving into people.
The Metropolitan Police say they are dealing with an incident on the bridge.
Transport for London said the bridge has been closed in both directions due to a "major police incident". Bus routes were being diverted.
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".
About five people were being treated for injuries after the vehicle mounted the pavement and hit them, she said.
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.