The Metropolitan Police said they are dealing with an incident at London Bridge amid reports of shots being fired.
The Metropolitan Police tweeted: "We are currently dealing with an incident at London Bridge. Please stand by for updates."
Various social media users have reported hearing gunshots from London Bridge.
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."
BBC correspondent John McManus told BBC News that he had heard several gun shots 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."
Political commentator Owen Jones tweeted: "I'm near London Bridge, looks like a bad incident - people running, police yelling 'keep moving', lots of emergency services, area cordoned off. Stay safe everyone."
"Police then quickly arrived, including armed police, and then a number of shots were fired at this man," Mr McManus told BBC News.
"Police have now cleared the bridge, everybody has been told to move back and now on the northern side of the bridge, but there are more shots going on.
"I can't see what's happening now because we've been moved out of the way but there are shots going on, and you probably hear the police sounds as well.
"Everybody has cleared the area, the entire bridge has been moved, and right on the northern part of the bridge, right up until quite near the Bank of England, is being cleared."
More to follow