Eyewitnesses say at least 12 people were injured on Westminster Bridge, while a Ferry appears to have crashed into the bridge and another vehicle appears to have crashed into a fence outside the Parliament.
Witnesses have said they heard sounds like gunfire.
MPs are being kept inside.
Police on scene have said it is a firearms incident.
Armed officers reportedly opened fire and shot a person outside the Houses of Parliament.
Pictures are emerging on social media
A number of shots were fired at around 2.35pm moments after a loud bang was heard inside the Palace estate.
According to eyewitnesses there was three bodies were visible on the floor.
The House of Commons has been suspended by Deputy Speaker Lindsay Hoyle.
Pedestrians stop to help injured person on Westminster Bridge
Gunfire at Westminster LIVE images: Attack outside Parliament Wed, March 22, 2017
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Reports of 12 people mown down on Westminster Bridge amid reports of gunfire
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Palace of Westminster incident
People have been seen fleeing the area.
The incident follows Prime Minister's Questions (PMQs).
A Met Police spokesman, said: "Police were called at 14.40 to a reports of an incident on Westminster Bridge.
“It’s being treated as a fire arms incident. Officers are on the scene and dealing.”