Pictures on social media showed several police cars on the bridge, with a lorry stopped across the central reservation of the dual carriageway bridge.
AThere are up to five casualties, police told Sky.
A Met Police spokesman said: “Police were called at 1.58pm to a stabbing at premises near to London Bridge. Emergency services attended, including officers from the Met and City of London Police.
BBC reporter John McManus says he saw a group of men on London Bridge and there “appeared to be a fight going on”.
Armed police then quickly arrived and shots were fired.
One video shows an officer near the northside of the bridge crouched behind a police car aiming a gun down the bridge.
The video then moves to what appears to be a body.
Sky’s home affairs correspondent Mark White said: “An attacker, we believe armed with a knife, started attacking people and that armed police were deployed. We believe that alleged attacker has now been fatally shot.”
People near the scene have been sharing videos on Twitter.
London Ambulance Service are now on the scene.
More to follow