Screams rang out as a grey SUV mounted the curb on Westminster Bridge and headed directly towards a group of tourists taking selfies in front of Big Ben.
The Hyundai didn't slow but ploughed into at least a dozen people, sending them flying like skittles, according to witnesses.
One woman leapt off the bridge into the Thames to avoid being hit but others lay strewn across the scene - many bleeding, some unconscious, three dead.
The atrocity is being labelled a terror attack and has rocked England's capital, a city where millions of people already live their lives under a daily terror alert threat.
Michael Adamou, 25, was driving the 453 bus when he said he saw the car speeding in the direction of London's Houses of Parliament at 2.40pm on Wednesday UK time.
"The first thing I saw was the two people laying lifeless on the floor. They were on the pavement.
"I heard one guy come running behind me shouting his wife had jumped into the river to avoid getting knocked down."
Witness Rob Lyon, 34, said: "I heard a wheel definitely hit a kerb and quite a large crunch noise, I looked up and saw a car clearly hitting people as it came towards me.
"I just instinctively jumped off the pavement. I just saw people being hit by a car at speed, it was harrowing."
The car only came to a stop when it crashed into bollards just metres from Parliament, which was sitting at the time.
But the terror did not stop there.
The driver of the car, believed to be an Asian man in his 40s, leapt out and ran through the gates of the Palace of Westminster where he repeatedly stabbed unarmed police officer Keith Palmer who had 15 years of service.
The terrorist, a thick-set man dressed in black, started running towards the entrance of Parliament ignoring the shouts of police officers behind him.
When he was about 45m from the entrance shots rang out. Two plain-clothes police officers opened fire and brought the attacker to the ground.
Witness Rick Longley described the moment the attacker began stabbing a policeman.
"A guy came past my right shoulder with a big knife and just started plunging it into the policeman."
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 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.
"He was running through those gates, towards Parliament, and the police were chasing him."
Paramedics fought to save the life of the police officer and his attacker on the ground of the cobbled courtyard just yards from the House of Commons, but to no avail.
Doctors at St Thomas' Hospital at the south end of the bridge raced out to help those mowed down on the bridge and described finding "catastrophic injuries".
At least 40 people were seriously injured, including three police officers on their way back from a commendation ceremony and three French teenagers on a school visit to London.
As the attack unfolded Parliament went into lock-down and Prime Minister Theresa May was rushed to Downing St where she chaired a meeting of the government's emergency Cobra committee to discuss the immediate response to the attack.
Speaking later, May vowed Britain would "never give in to terror" and "defeat hate and evil" after she blasted the "sick and depraved" attack in Westminster.
She added the "forces of evil would never drive Britain apart" and praised police and security staff who "ran towards danger even as they encouraged others to move away".
New Zealand-born British MP Sir Paul Beresford said MPs were pouring in to vote when the building was locked down.
One part of the tragedy had been playing on his mind: The death of the 49-year-old police officer.
Sir Paul knew Palmer, a husband and father, and he also knew the security committee had been considering making a change at the gates which may have prevented the assailant coming through.
"Perhaps if we had got the gate, this guy, he might not have gotten up [there]."
Despite the day's events Sir Paul said the work of Parliament would continue as normal.
"Tomorrow at the usual time the House of Commons and the House of Lords will open and we will not let the terrorists win."
Other Kiwis also described their experiences as the day unfolded.
Auckland woman Natassia Snell, 25, worked around the corner from the Westminster Bridge at a hedge fund firm which was put into lock down.
"There are lots of sirens going on and they've closed off some streets and redirected all the buses."
Snell usually tried not to think about what could go wrong in London.
"You know London is a red level terrorist alert. But if you did think about it, how would you do anything?"
Kiwi Juliet Brine works about a kilometre from Parliament.
She and her colleagues were in their offices when they heard "a lot of commotion" and saw helicopters and emergency responders.
She felt rattled but was reassured by the reaction of those in the city.
"Everyone has been so calm. Typical Londoners."
The attacker's identity is yet to be revealed, along with the names of his other victims.