There are reports a man has jumped or fallen from the bridge into the River Thames.
One Twitter user said: "Somebody has jumped from Westminster bridge. Huge rescue operation."
Another said: "Something is definitely going down in #London. Helicopters out now.
Emergency services at Westminster Bridge this morning
"Police and rescue swarming to #WestminsterBridge."
A third said: “Watching the scenes at Westminster Bridge atm, it's reassuring to see how quickly and meticulously the emergency services have responded.”
Firefighters look for a man in the water from Westminster Bridge
The Met Police said: "Emergency Services are dealing with reports of a man in the water near Westminster Bridge #London
"This is not being treated as suspicious".
They said the man had not yet been located in the river.
A police officer on Westminster Bridge this lunchtime
Several ambulances are at the scene this morning, just days after a deadly terror attack took place on the bridge.
Police divers are also at the scene.
A man is believed to have fallen or jumped into the Thames from Westminster Bridge
A vigil in memory of the four victims of last week's terror attack is due to take place on the Bridge in the next few hours.
This is a breaking story. More to follow…