Emergency services have been called to London Bridge amid reports of shots being fired.
The Metropolitan Police said: “We are currently dealing with an incident at London Bridge. Please stand by for updates.”
The London Ambulance Service said it had crews at the scene.
BBC correspondent John McManus told BBC News that he had heard several gunshots on London Bridge.
“Just a few minutes ago I was walking across London Bridge on the south bank to the north bank of the bridge,” he said.
“There appeared to be a fight going on on the other side of the bridge, with several men attacking one man.”
Various social media users also reported hearing gunshots in the area.
One video shows what appears to be police pulling equipment out of a vehicle nearby.
Those in the video can be heard saying: “They’ve actually got guns out. They’ve got the guns out.”