In latest developments: Police name fatally stabbed officer, pedestrian death toll rises to five and killed suspect's identity known but not released.
KEY POINTS
London police have named officer Keith Palmer, 48, as the victim of a car ramming and knife terrorist stabbed outside the Houses of Parliament.
Police have indicated they know the attacker but are not yet ready to name him.
In further developments, police say three other people died in addition to the attacker and at least 40 others were injured. These now include five South Korean tourists.
Metropolitan Police Assistant Commissioner Mark Rowley, who heads the national counter-terrorism unit, says it is believed the attacker was inspired by international terrorism.
Prime Minister Theresa May describes as "sick and depraved." Her comments have been echoed by leaders around the world, including New Zealand Prime Minister Bill English.
First attack since 2005
This is the first terrorist attack on UK soil since 2005 when coordinated bombings by Islamist extremists on buses and subway trains claimed 52 lives. Authorities claim to have foiled dozens of subsequent planned attacks.
One of the dead is the so far unidentified suspect, who drove into pedestrians on Westminster Bridge at about 2.45pm local time before trying to break into the grounds of Parliament and fatally stabbing a policeman.
The man was shot by police and later died after being taken away in an ambulance.
Two died in the car attack and 20 were injured, with 12 being admitted to hospital with serious wounds. Two are in a critical condition and the others are described as stable.
The injured include four students from Edge Hill University, Lancashire, and three French schoolchildren.
One injured woman was pulled from the Thames River with injuries at about 5pm.
Witnesses described the attacker as Asian. He was pictured being treated by paramedics on a stretcher, as two knives used in the assault lay on the ground nearby.
Paramedics fought to save his life and that of his police victim on the floor of the cobbled courtyard in front of Parliament, with Foreign Office minister Tobias Ellwood among those who rushed to help.
The police officer was wheeled away on a stretcher with his face covered and it was later confirmed he had died.
Prime Minister's reaction
Theresa May describes the attack on the heart of the capital as "sick and depraved," adding that attempts to defeat UK values are "doomed to failure."
Speaking in Downing Street after chairing a meeting of the government's Cobra emergencies committee, Mrs May paid tribute to the "exceptional men and women" of the police force who responded to the attack.
"We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart," she said.
"The location of this attack was no accident. The terrorist chose to strike at the heart of our capital city where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."
Around 1000 people in P{arliament grounds at the time were taken to Westminster Abbey for safety and were then processed by police
MPs were locked in the House of Commons for more than four hours and business was suspended But the House of Commons and Lords will sit at their usual times on Thursday
Earlier report:
In a police briefing, Assistant commissioner Mark Rowley, who is head of counterterrorism, confirmed that four people have died including one police officer and the attacker.
The attacker ploughed into pedestrians on Westminster Bridge before crashing into the railing of the Houses of Parliament and exiting the car to continue the attack.
He then stabbed one of the armed officers protecting parliament.
Police are working on the assumption that there was only one attacker.
Two people died on Westminster Bridge.
At least 20 people have been injured, including three police officers who were returning from a commendation ceremony.
Mr Rowley says the public should expect a heavier police presence on the streets of London in forthcoming days. He said:
“Please be vigilant you will see more police officers armed and unarmed on the streets of London,” he says.
Witnesses identified the car as a grey Hyundai i40.
Earlier, Parliament was suspended and all members, staff and visitors evacuated after a lockdown.
Prime Minister Theresa May was ushered away from Parliament after gun shots broke out after the attack. She then chaired a meeting of the government's emergency Cobra committee.
