Developing:

There are reports of shots being fired outside the Houses of Parliament and the Palace of Westminster is reportedly in lockdown.

Politicians and journalists have tweeted about hearing loud crashes outside the buildings, as witnesses said they saw people being treated for injuries.

Witnesses have reported seeing a man with a knife within the palace grounds.

A police officer has also told the BBC’s political editor Laura Kuenssberg that someone has been shot outside Portcullis House.

We were called at approx 2:40pm to reports of an incident at #Westminster Bridge. Being treated as a firearms incident – police on scene

— Metropolitan Police (@metpoliceuk) March 22, 2017

We’ll update this article as we have more information.

https://twitter.com/IsraelHatzolah/status/844569248345341952

Injured people are seen on Westminster Bridge in London after a major incident. Keep up to date with our live blog: https://t.co/rqYw0ZjWh5 pic.twitter.com/ZlPRlH8MF4

— David Sim (@davidsim) March 22, 2017

BREAKING: London police says they are treating Westminster attack 'as terrorist incident until we know otherwise'

— The Associated Press (@AP) March 22, 2017

#LondonAttack update: Reports of civilians 'mowed down' on Westminster Bridge; Air ambulance on scene [pics, video] https://t.co/Gq6YvVNZGv pic.twitter.com/sdjeLV6Ne3

— Twitchy Team (@TwitchyTeam) March 22, 2017

It appeared that the man was attacking a police officer, then advanced into New Palace Yard. Armed officers moved in quickly, and shot him.

— Charlie Cooper (@CharlieCooper8) March 22, 2017