Tributes have poured in for the policeman killed in the Westminster terror attack, which left a total of four dead and 20 injured.

The unnamed officer was stabbed to death by an attacker as he guarded Parliament at around 2.40pm this afternoon.

Scotland Yard’s Head of Counter Terrorism Mark Rowley said the officer died protecting the public and his fellow officers.

In a statement released after the attack, Mr Rowley said the Metropolitan Police had “lost one of our own as he acted to protect the public and his colleagues”.

The Acting Deputy Commissioner added: “This is a day we planned for but hoped would never happen.

“Sadly it’s now a reality. We will continue to do all we can to protect the people of London.”

The scene where the police officer was killed in the grounds of Parliament (PA)

Steve White, chair of the Police Federation of England and Wales, described the attack as shocking and said it demonstrated the dangers officers face.

“No words can capture how members of the policing family will feel after today’s horrific events,” he said.

Westminster terror attack: Timeline of events

“This incident highlights the very real risks that police face each and every day.

“Officers will tonight take the opportunity to hug loved ones and seek comfort in the company of friends and family.

“But one will not. The pain of that officer’s family, friends and colleagues will be shared by us all.”

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Among the international messages of solidarity was a tweet from the French National Police, which voiced its support “to our British colleague @MetPoliceUK stabbed in the attack near Parliament”.

Several other forces issued messages, including the Los Angeles Police Department, who said their “thoughts & prayers are with the people of #London”.

Eyewitness Rick Longley described the knife attack: “We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman.