Five people died and nearly 40 others injured when a knife-wielding man rampaged through a street in his car and tried to storm the British symbol of democracy in London, hitting pedestrians and stabbing a police officer near the Parliament in an "Islamist-related terrorism" attack.
The dead included the attacker, who was "known" to the Metropolitan Police, and the 48-year-old police officer he stabbed. The police identified the officer as Keith Palmer.
Three civilians were killed when the man plowed his grey Hyundai i40 through the pedestrians on Westminster Bridge just outside the British Parliament.
The knife-man then tried to gain entry to the Parliament via the main entrance and stabbed a police officer.
Plainclothes armed officers shouted warnings at him before shooting a number of rounds.
He was later seen covered by blankets, presumed dead.
Dozens of injured people were rushed to hospital from the scene and one woman was confirmed dead by doctors at St Thomas Hospital near Parliament hours later.
UK Foreign Office Minister Tobias Ellwood administered mouth-to-mouth resuscitation to the stabbed police officer but said he "died at the scene".
The first mass casualty terrorist strike on Britain in over a decade was condemned as "sick and depraved terrorist attack" on democratic values by Prime Minister Theresa May.
The Scotland Yard has urged public to "remain vigilant" and said a review of policing strategy in London was underway, with additional policemen being deployed across the city.
Mark Rowley, Britain's national head for Counter Terrorism Policing and the acting deputy commissioner, has refused to comment on the identity of the attacker, but said "our working assumption is he was inspired by international terrorism" and and "Islamist-related terrorism".
"We are forensically examining a complicated crime scene that covers a wide area," Rowley said in a statement about the Met department's "fast-time examination" of the attack.
The lock down on Parliament, for several hours because of fears there could have been further attacks, has been lifted.
The UK threat level has been at 'Severe' for some time and the police said they were not considering to change the level.
The Indian High Commission in London has set up a special "Public Response Unit" for any Indians caught up in the terror attack. .
The incident happened on the first anniversary of attacks in Brussels by Islamist militants that killed 32 people, and the use of a vehicle was a copy of an Islamic State tactic used previously in the French city Nice and Germany's Berlin.
French Prime Minister Bernard Cazeneuve said French school pupils were among the injured and offered "solidarity with our British friends, and full support" for the students and their families.
London Ambulance said they were "working closely with other members of the emergency services at the scene" to ensure patients quickly receive the medical help they need.
An air ambulance was deployed to the scene to remove the casualties.
Earlier, Commons Leader David Lidington, who suspended the Parliament session as the attack unfolded outside, told lawmakers the "alleged assailant was shot by armed police".
May, who had just completed her weekly Prime Minister's Questions, was seen being ushered into a car as gunfire rang out at Parliament during the incident.
Downing Street said the prime minister was "safe" as an emergency services helicopter was seen landing in Parliament Square, which was cordoned off and closed to traffic.
"The Prime Minister was brought back to Number 10 from Parliament. She is currently monitoring the situation," said a Downing Street spokesperson.
May chaired a meeting of the government's emergency Cobra committee at Downing Street.
Lawmakers and peers were in lock down inside the Parliament building for an extended period before being walked down across the road to Westminster Abbey.
Workers and members of public in buildings in surrounding areas were also held up behind the cordoned off area. A number of schoolchildren were also held back until the police cordon was lifted. Tourists on the famous London Eye attraction were among those trapped in the pods as part of the lockdown.
There was chaos in the area as people ran amok and were pushed back by Scotland Yard officers.
In a statement, Opposition Labour leader Jeremy Corbyn said: "Our thoughts are with the victims of this horrific attack, (their) families and friends."
London mayor Sadiq Khan said his thoughts were with those affected and expressed his gratitude to the city's police and emergency services who "work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances".