BBC reporter John McManus said he saw a group of men involved in a fight on the bridge.
Police then arrived and shots were fired, he said.
McManus told the BBC News channel: “Just a few minutes ago I was walking across London Bridge on the south bank to the north bank of the bridge.
“There appeared to be a fight going on on the other side of the bridge, with several men attacking one man.
“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,” he added.
“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.”