Eyewitnesses spoke of their shock as a car mowed down pedestrians on Westminster Bridge before crashing into railings outside the Houses of Parliament, where a man leapt out with a knife and stabbed a police officer.
Gunfire rang out as armed police fired on the attacker, who later died after a bloody rampage which killed four people, including the police officer, and wounded at least 40 others.
Lawmaker Mary Creagh, who was confronted by crowds and armed guards on her way to parliament, spoke of "a real sense of panic".
"I was met by about 40 people running towards me saying: 'Get back, get back, there's been a shooting!'" she told reporters.
Police cars and ambulances flooded the area of the attack - one of several guarded entrances to Houses of Parliament, a warren of rooms and staircases which was later evacuated.
An air ambulance was also at the scene, while police taped off the area.
"It was very, very frightening," Creagh said.
'Shocked and dazed'
Foreign Office minister Tobias Ellwood was pictured giving first aid to an injured police officer.
MP Damian Collins said he heard shouts and gunshots before being told by police to get on the ground along with around 30 other lawmakers, where they waited for a minute before being told to go to their offices in Portcullis House.
"When the incident happened they (police) were very quick, very professional and very brave. They did what they are there for - to protect the people who work in the Palace of Westminster," he told AFP.
Police did not identify the other two people killed in the attack - one of whom was reportedly a woman - and said "at least 20" others were wounded, among them three police officers.
One injured woman was pulled from the River Thames, and three French school children were also hurt, officials in Paris said.
A junior doctor quoted by local media said they were treating "catastrophic injuries".
"There's a lot of people who feel very very shaken up," said Liberal Democrat leader Tim Farron who was evacuated along with a number of other MPs to the nearby Metropolitan Police headquarters.
A staff member in one of the buildings close to the Houses of Parliament told AFP they were "shocked and dazed" by the attack.
Video: Witnesses describe attack
'A lot of shouting'
Kirsten Hurrell, 70, who owns a news-stand at the scene, said she heard a car crashing into fencing surrounding parliament and initially thought there had been an accident.
"I heard a couple of shots which I now know are gunshots so I began to realise it was something a little more serious," she told AFP.
"There was a lot of shouting going on... and a lot of police presence coming from everywhere."
Richard Jones, a 31-year-old bar manager, who reached the scene just after the attack said he saw police cars and ambulances arriving along with an initial six police officers.
"After a few seconds, another 25 police officers in full tactical gear with helmets and body armour, and then they stormed into the Houses of Parliament," he told AFP.
"Then they started to evacuate the MPs in black unmarked police cars."
Also seen driving away was Prime Minister Theresa May as she prepared to chair a meeting of the government's COBRA emergencies committee.
View from the Eye
Tourists who usually flock to the area for a glimpse of London's iconic Big Ben clock tower looked on as heavily-armed police officers cordoned off the roads.
Buses on Westminster Bridge were evacuated and the London Eye was also eventually emptied of visitors.
Jack Hutchinson, a 16-year-old American, said he spent three hours stranded on the London Eye with his parents.
Once the carriage moved, he looked down and "saw three bodies lying on the ground, and a whole lot of police. It was pretty terrifying," he told AFP.
"I don't think I'll go on the London Eye again. But I thought that London was a great place and I don't think that can be spoiled because of one incident," he added.
