By Angela Dewan

CNN

LONDON (CNN) -- A police officer was stabbed and the assailant shot by police near the British Houses of Parliament, in what police are treating as a terrorist incident.

London's Metropolitan Police said on Twitter they were called at approximately 2:40 pm to reports of a firearms incident near Westminster Bridge.

British media reported there were multiple injuries.

London's Metropolitan Police said in a statement they were "treating this as a terrorist incident until we know otherwise."

Latest developments

Houses of Parliament, roads and Westminster underground station on lockdown. Images show a car crashed into the perimeter fence of Parliament. Members of Parliament locked in, told to stay away from windows. Police were called at 2:40 pm after reports of a firearms incident. Air ambulance at the scene to remove casualties.

The incident occurred as parliament was in session, and proceedings in the House of Commons were suspended.

Dozens of MPs remain in the House.

TV images have emerged of a car crashed into perimeter fence outside the Parliament building.

"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police," House leader David Lidington told MPs.

He confirmed that a serious incident had taken place "within the estate."

"An air ambulance is currently attending the scene to remove the casualties. There are also reports of further violent incidents in the vicinity of the Palace of Westminster but I hope colleagues on all sides will appreciate that it'd be wrong of me to go into further details here until we have confirmation from the police and from the House security authorities about what is going on."

Henry Mance, a political reporter for the Financial Times, told CNN that people inside have been told to stay away from windows.

Armed officers have been sent inside parliament.

Member of parliament Sir Gerald Howarth said the House was in the middle of a vote when the session was suspended and members locked in.

"It appears to be very, very serious indeed. The Leader of the House has told us it's been confirmed that one police officer here has been stabbed. It appeared that a car was coming towards the House of Commons mowing down pedestrians on the way and the driver then got access to the parliamentary estate stabbed a police officer and was shot," he said.