Gun fire has been heard outside Westminster after a man wielding a knife was seen in the area.
At least a dozen people have been injured, according to Reuters.
The Press Association said two people were seen lying within Old Palace Yard, immediately outside Westminster Hall.
Sky's Tamara Cohen said the Prime Minister was "bundled out by [a] plain clothes policeman" when the news broke.
The Huffington Post UK's Owen Bennet, who was in Parliament at the time, said:
"I heard what sounded like an explosion and raised voices outside Parliament, and so I rushed to the window.
"I saw people running past the entrance to the New Palace Yard entrance to Parliament, and then at least one person try to run into the Yard itself. A police officer chased this person and wrestled them to ground.
"Shots were then fired, but I can't remember how many and I didn't see who fired them.
"I ran down the stairs from my office overlooking the Yard to get more information, but the police prevented us from leaving the stairwell.
"When I returned to my office, I could see two people lying on the ground outside Westminster Hall, but neither of these were the person I saw wrestled down by the police officer minutes earlier."
Other political reporters based in the area shared their version of events:
The House of Commons has also reportedly been suspended.
An air ambulance has reportedly been scrambled to the scene.