Four people, including an armed police officer and the lone attacker, have died in a terror attack near the Houses of Parliament in London.
Two of those killed were among more than 20 pedestrians hit by a car on Westminster Bridge, before it crashed.
The man stabbed the officer outside Parliament before being shot dead.
PM Theresa May said it was a "sick and depraved" attack on the heart of the capital and attempts to defeat UK values were "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 prime minister added: "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."
The attacker has not yet been identified.
Acting Deputy Commissioner and head of counter terrorism at the Metropolitan Police, Mark Rowley, said a major investigation was under way into the "marauding terrorist attack".
He said emergency services were called at 14.40 GMT after the car was driven over Westminster Bridge, hitting and injuring a number of members of the public and three police officers, who were on their way back from a commendation ceremony.
After the car crashed into the railings of the Houses of Parliament, a man armed with a knife "continued the attack" and tried to enter the building.
Eyewitnesses said police fired three or four shots as the knifeman lunged towards a second officer.
In latest developments:
- The prime minister said the UK terror threat level would remain at severe - its second highest - meaning an attack is "highly likely"
- Westminster underground station was shut and remains open for interchange only
- Home Secretary Amber Rudd urged everyone to remain calm but be vigilant and if they see anything they are concerned about report it to the police
- A group of French schoolchildren were on the bridge and three were injured
- 13 students from Edge Hill University in Lancashire were also caught up in the incident - two were taken to hospital and described as walking wounded; two others had minor injuries
- There are two police casualty bureau numbers: 0800 056 0944 and 0207 158 0010 for people worried about family and friends, or eyewitnesses
- Kings College Hospital says eight patients are being treated there - six male, and two female. Two are critical and two are stable
- St Thomas' Hospital said two patients had been admitted - both are stable
- Around 1,000 people were taken to Westminster Abbey for safety and were then being processed by police
- MPs were locked in the House of Commons for more than four hours and business suspended
- The House of Commons and Lords will sit at their usual times on Thursday
- Over the following days there will be extra unarmed and armed officers on the streets of London
- Paris mayor Anne Hidalgo says the Eiffel tower will go dark at midnight in homage to the London victims
- London mayor Sadiq Khan praised citizens and emergency services for their "tremendous bravery" and said: "Londoners will never be cowed by terrorism."
Witnesses say there was what appeared to be a warning shout and then the crack of three or four shots before the attacker fell to the floor.
Foreign Office minister Tobias Ellwood - a former Army officer whose brother died in the Bali terrorist bombing in 2002 - attempted mouth-to-mouth resuscitation of an injured police officer.
The Port of London Authority said a woman was pulled alive from the River Thames near the bridge and was being treated for serious injuries
Source: BBC