The attacker mowed down several pedestrians as he drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings then running through the gates of the Palace of Westminster and stabbing the officer.
At least 20 people were injured when the car mowed them down on a central London bridge.
The knifeman was shot by police in the shadow of Big Ben, as he had tried to force his way into a courtyard just outside the Houses of Parliament.
Conservative parliamentarian Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed mouth-to-mouth resuscitation on the police officer who was stabbed and later died. About 10 metres away from the police officer was the attacker who was shot dead by police after scaling the security wall toward the Parliament's grounds.
Ellwood, who served in the British military and served in Bosnia, Northern Ireland, Kuwait and Cyprus, applied pressure to the police officer's multiple lacerations.
Photographs showed Ellwood's bloodied hands and face from the police officer's wounds while the alleged attacker was seen nearby.
It is believed there is only one assailant, but a thorough search of the area is underway.
London mayor, Sadiq Khan, says he's grateful the police and emergency services could show "tremendous bravery in exceptionally difficult circumstances," and additional officers will be on patrol tonight to ensure the safety of Londoners.
One woman was pulled from the Thames river with serious injuries. It's not yet known whether she jumped into the river or was thrown in after being struck by the vehicle.
The Port of London Authority has confirmed that she was pulled from the river alive, requiring urgent medical attention. They said she was the only person in the river they were aware of.
An eyewitness described the moment he realised a terror attack was going on.
"A guy came past my right shoulder with a big knife and just started plunging it into the policeman," he says.
Daily Mail journalist Quentin Letts says he watched the assailant run towards parliament after stabbing the officer.
"He disengaged and ran towards the House of Commons entrance used, got about 20 yards or so when two plain-clothed guys with guns shot him,” Mr Letts says.
Two women witnessed the attack from on-board a double-decker bus.
"We heard what we thought were gunshots and then we saw this car just plow into this body," they told Channel 4 News.
"The body was trapped right underneath the front wheel. And then as the bus was pulling in slowly, just saw all these bodies that were spread out all the way along… from one end of the bridge to the other. It was horrendous. Absolutely horrendous."