FOUR people, including an unarmed police officer, have been killed after a knifeman brought terror to the heart of Westminster.

At least 40 other people were hurt, with some said to have suffered "catastrophic" injuries.

The attacker, armed with two large knives, mowed down pedestrians with his car on Westminster Bridge, including schoolchildren, then rushed at the gates in front of the Houses of Parliament, stabbing the policeman, who was unarmed, before being shot dead by other officers.

Prime Minister Theresa May condemned the attack in Westminster as "sick and depraved".

Speaking outside 10 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.

She confirmed that a single attacker drove a car into pedestrians walking on the bridge, killing two of them, before running with a knife through the front gates of the Palace of Westminster.

She vowed that "the forces of evil" would never be allowed "to drive us apart".

The police officer who died was named by Scotland Yard as Keith Palmer, a 48-year-old husband and father.

Eyewitnesses described scenes of terror when gunfire rang out as the man approached a second officer.

Scotland Yard's top anti-terror officer Mark Rowley said the two other fatalities were on the bridge when the terrorist struck.

Mr Rowley told reporters outside New Scotland Yard, just yards from the scene of the attack: "This was a day we've planned for but hoped would never happen. Sadly it is now a reality."

The PM was seen being ushered into a silver Jaguar in the grounds of the palace as what sounded like gunfire rang out at around 2.45pm.

Two people were seen to be lying within Old Palace Yard, immediately outside Westminster Hall.

One of those confirmed injured on Westminster Bridge was a student from Lancashire.

Four students from from Edge Hill University, in Ormskirk, were injured as they were caught up in the attack on the bridge.

They were among a group of 13 students and a lecturer who were on the first day of a two-day educational visit to the Houses of Parliament.

The students had just left Parliament and made their way on to Westminster Bridge after they had watched Prime Minister's Questions and met with MPs.

The sitting in the House of Commons was suspended while police officers sealed off the area around the incident.

Foreign Office Minister Tobias Ellwood has been branded a "hero MP" after attempting to save the life of the police officer killed in the Westminster terror attack.

The former soldier gave mouth-to-mouth resuscitation and attempted to stem the flow of blood amid the carnage in New Palace Yard.

Immediately before the incident, at around 2.45pm, a crowd of passers-by was seen running from the direction of Westminster Bridge and around the corner into Parliament Square.

Staff inside Parliament were told to stay inside their offices.

A counter-terrorism investigation is already under way, police said.

Giving a statement outside Scotland Yard, police spokesman Commander BJ Harrington said: "A senior national coordinator has declared this a terrorist incident.

"Although we remain open-minded to the motive, a counter-terrorism investigation is already under way led by the Met counter-terrorism command.

"We received a number of different reports which included a body in the river, a car in collision with pedestrians and a man armed with a knife.

"Officers were already in the location as part of routine policing but immediately additional officers were sent."

MPs were locked in the chamber of the House of Commons for almost five hours after business was suspended following the terror attack in Westminster.

Only after dark had fallen was a security lockdown lifted, allowing hundreds of MPs to leave the building.

Deputy Speaker Lindsay Hoyle halted the sitting of the House as MPs were preparing to vote on pensions legislation, telling them to remain in the chamber as details of the incident filtered through.

Bolton and Bury MPs were among those put into lockdown in Parliament.

Sir David Crausby, MP for Bolton North East, said: "It is very tragic and I really feel for the families of those who have been killed or injured.

"MPs have been inconvenienced, but that is nothing compared to what those families have suffered.

"My heart goes out to them, particular to the family of the police officer.

"The police here dedicate their lives to protecting our democracy and we have to be immensely grateful to them — not only for keeping us safe, but also our democratic institutions."

Yasmin Qureshi, who represents Bolton South East, said: "I was in the members' lobby waiting for a vote and then we were suddenly put into lockdown.

"We couldn't hear any of what happened from where we are, but one of my staff who was in my office at the time says he heard four gunshots outside.

"Sittings have been suspended for the day and people have been told to stay in their offices.

"Our thoughts are with those people who have been caught up in the incidents outside.

"Everyone here is very concerned, but is trying to remain as calm as possible."

Chris Green, MP for Bolton West, said shortly before 6pm that those in Parliament had been told that they would be held at the crime scene as potential witnesses for another 60 to 90 minutes.

He said: "The police are being meticulous in gathering evidence and making sure everyone has the opportunity to say what they saw.

"It is a terrible situation for the family and friends of those who have died, and our thoughts are with them.

"It shows that the police officers that I walk past on the way in every day are really putting themselves on the front lines. The officer who has died was doing his duty in protecting people."

David Nuttall, MP for Bury North, said: "We were in the middle of a select committee, agreeing a report that’s going to be coming out shortly. The bells went for a division.

“Whilst the doors were still open the deputy speaker suspended the sitting right in the middle of the division and told everyone to stay where they are in the chamber.

 "We've just been told the house has now scrapped it's sitting for the rest of the day.

"I don't know how long we have got to stay here for. We were in the middle of business. It was scheduled to be a bill to do with pension schemes, then there was to be a debate on international trade following the exiting for the EU., what we were going to do about international trade.

"It was planned to be a big debate until 7pm.

"The mace again so the house is no longer sitting, we can't go anywhere. I can't even go up to the committee room."

Ivan Lewis, MP for Bury South, was not in Parliament as he was attending a funeral in Manchester.

Mike Kane MP, Wythenshawe and Sale East, said: “The overwhelming feeling in the chamber is one of concern and sympathy for the victims -

“The business for the day is obviously suspended and the deputy speaker Lindsay Hoyle is keeping us updated.

“The security incident happened as we were voting so majority of MPs in the chamber/voting lobby now.”

Greater Manchester Police says it is reviewing security in the region.

Chief Constable Ian Hopkins said: “I wish to extend the thoughts and sympathy of all at Greater Manchester Police to the families of the Metropolitan Officer and people murdered during the terrorist attack in London this afternoon.

“Police officers recognise the risks to themselves in undertaking their role to protect fellow citizens. Despite the knowledge of the risk we face, it is absolutely shocking when an officer carrying out their public duty is murdered in this way.

“Greater Manchester Police are reviewing with immediate effect officer deployments across the region whilst we find out more about today's attacks.”