According to Sky News, the Parliament is in lockdown after a police officer was stabbed and a dozen people mowed down on Westminster Bridge.
Commons leader David Lidington said a "police officer has been stabbed" and the "alleged assailant was shot by armed police" following a "serious incident".
After the incident pictures emerged of a man lying at the bottom of some stairs next to the Palace of Westminster.
There are also reports of a dozen people injured on Westminster Bridge after a car attempted to run down a number of pedestrians.
Staff inside Parliament have been told to stay inside their offices.
An air ambulance landed in Parliament Square minutes after the incidents and medics were treating two people at the scene.
Mr Lidington said: "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 until we have confirmation from the police and from the House security authorities about what is going on."
Witnesses described seeing a middle-aged man carrying a knife and running.
Jayne Wilkinson said: "We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy (British press' codeword for Muslim) in about his 40s carrying a knife about seven or eight inches long.
"And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.
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