Metropolitan Police told the Evening Standard the bridge was shut off this afternoon.
Officers were called shortly before 1.30pm on Thursday and the crossing remains closed at both ends.
Pictures show ambulances and police vehicles parked in the centre of the crossing with several boats positioned below in the River Thames.
One person said on Twitter: “Westminster Bridge shut, heavy police presence.”
Another added: “Lots of police activity on the bridge and in the river.”
— Sam (@SamFromHK) September 9, 2021
Londoners stood behind the cordon at the entrance to the bridge as uniformed officers gathered to one side in the distance.
A Met spokesperson said: “At approximately 1.25pm on Thursday September 9, police were made aware of a man threatening to self-harm on Westminster Bridge.
“Emergency services are in attendance.”
The incident remains ongoing.
: Anyone seeking help can call Samaritans free on 116 123 or visit Samaritans.org