LONDON - Witnesses said the attacker was apparently a "thick-set man" wearing black clothes who made his way through the gates of London's parliament grounds, just below the famed Big Ben.
He had something in his hand, resembling "a stick of some sort", said Mr Quentin Letts, UK tabloid Daily Mail's parliamentary sketchwriter who witnessed the drama that unfolded yesterday afternoon in one of London's busiest areas.
"He was challenged by a couple of policemen in yellow jackets," Mr Letts told the BBC. Then, one of the policemen fell to the ground. "We could see the man in black moving his arm in a way that suggested he was stabbing or striking the yellow-jacketed policeman," he said.
The other officer ran for help and the assailant ran about 10 metres towards the entrance, before two plain-clothed policemen shouted a warning at him. When he ignored them, they fired at him two to three times and he fell.
Press Association political editor Andrew Woodcock who saw what happened from his office window overlooking New Palace Yard, which lies to the northwest of the Houses of Parliament.
"I heard shouts and screams from outside and looked out, and there was a group of maybe 40 or 50 people running round the corner from Bridge Street into Parliament Square. They appeared to be running away from something," he said.
"As the group arrived at the Carriage Gates, where policemen are posted at the security entrance, a man suddenly ran out of the crowd and into the yard. He seemed to be holding up a long kitchen knife," he told the BBC.
"I heard what sounded like shots - I think about three of them - and then the next thing I knew there were two people lying on the ground and others running to help them.
"Armed police were quickly on the scene and I heard them shouting to people to get out of the yard."
Eye witnesses at the nearby Westminster Bridge said they saw an SUV mow down at least five people.
Mr Radoslaw Sikorski, a senior fellow at Harvard's Centre for European Studies, posted a video on Twitter which showed at least two people lying on Westminster Bridge. "A car on Westminster Bridge has just mowed down at least five people," he wrote.
Ms Kirsten Hurrell, who witnessed the incident from her newspaper kiosk in Parliament Square told The Guardian that the car had come to a halt following shots.
"There were shots to stop the car. The car turned, went completely over the cycle lane and rammed nose first into the parliamentary fence. There was someone on the ground on the right of the car. They didn't look very good," she said.
Mr Toby Helm, UK newspaper The Observer's political editor said in a tweet that he could see people stranded on the giant ferris wheel, London Eye, on the other side of Westminster Bridge, after it stopped running.
The attraction tweeted to say it was "holding all of our guests within our attractions as per tried and tested security procedures".