The death toll and the number of injured has increased over the evening.
Here’s what we know at the end of a tragic day in the centre of London:
An attacker, armed with two large knives, drove a car into pedestrians – including schoolchildren – on Westminster Bridge.
He then rushed at the gates in front of the Houses of Parliament and stabbed a policeman before being shot dead by other officers.
How many casualties are there?
Scotland Yard’s top anti-terror officer Mark Rowley confirmed that five people have now died and about 40 people have been injured.
One of them is the police officer who was stabbed, who has been named as 48-year-old husband and father Keith Palmer. Scotland Yard also clarified that he was not armed.
Another is his attacker, and there are also three members of the public who have lost their lives.
Do police know who the attacker is?
Rowley suggested that police think they know who the attacker is.
He also revealed that their assumption is that the attacker is linked to Islamic State.
What has Theresa May said about it?
The PM condemned the “sick and depraved” attack as she stood outside 10 Downing Street after chairing a meeting of the Government’s Cobra emergencies committee.
She paid tribute to the “exceptional men and women” of the police force who responded to the attack and vowed that “the forces of evil” would never be allowed “to drive us apart”.
What about Sadiq Khan?
The London Mayor released a video telling residents and visitors of London to not be alarmed, stating: “Our city remains one of the safest in the world.”
He also vowed that “Londoners will never be cowed by terrorism.”
How have other world leaders responded?
Canada PM Justin Trudeau said he was “shocked and saddened” to learn of the “cowardly” attack.
US President Donald Trump has offered the “full co-operation and support” of his country in “bringing those responsible to justice”.
President of Ireland Michael D Higgins said he wanted to express “the sympathy, condolences and solidarity of the Irish people with our neighbours at this difficult time”.
What happens next?
The attack triggered a response police hoped would never be needed – but which has been rigorously planned and rehearsed, even as recently as Sunday.
In the aftermath over the next few days, the public have been told to expect more armed and unarmed police on the streets in the capital.
But other forces around the UK will also be reviewing their arrangements in light of the attack.