FOUR people have died and at least 20 others have been injured after a terror attack in Westminster.

A police officer who was stabbed at the Houses of Parliament and his attacker are among the dead, according to Metropolitan Police Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley.

He said: "Sadly I can confirm that now four people have died and that included the police officer who was protecting Parliament and one man we believe to be the attacker who was shot by a police firearms officer.

"The officer's family have been made aware.

"At least 20 people have been injured."

He made a statement outside Scotland Yard to update the public about the incident, the ongoing operation into what has happened and what people can expect in the coming days.

He said: "As you know, we've declared this as a terrorist incident and the counter-terrorism command are carrying out a full-scale investigation into events today.

"The attack started when a car was driven over Westminster Bridge, hitting and injuring a number of members of the public also including three police officers on their way back from a commendation ceremony.

"The car then crashed near to Parliament and at least one man, armed with a knife, continued the attack and tried to enter Parliament.

"Sadly I can confirm that now four people have died - that includes the police officer who was protecting Parliament and one man we believe to be the attacker who was shot by a police firearms officer. The officer's family have been made aware. At least 20 people have been injured.

"As part of long-established and well-rehearsed plans Parliament has been locked down and the Met responded in line with our plans for a marauding terrorist attack. That response included uniformed and specialist firearms officers.

"We now of course have an ongoing operation and, whilst we currently believe there was only one attacker, I'm sure the public will understand us taking every precaution in locking down and searching the area as thoroughly and exhaustively as possible.

"I know officials working in Parliament and the public in the area completely understand the need to do this and I thank them for their patience and their support.

"This investigation has the full weight and expertise of the counter-terrorism command behind it and if there are people who saw the events unfold today that haven't yet spoken to us I would urge to get in touch."

"A crime scene will remain in place in the affected areas in Westminster and you will understand it is vital as we carry out a painstaking investigation to recover all the possible evidence.

"Looking forward, throughout the rest of the day, including when people are commuting home and indeed over the days that follow, the people of London will see extra police officers armed and unarmed on our streets.

"This includes our officers working longer hours and extra shifts alongside our colleagues from British Transport Police and the City of London, and of course as you're aware we can call on the support of the military should we need to at a future point.

"We are also in a process of opening up our casualty bureau to help those people who are worried about friends or family who may have been caught up in the attack.

"Furthermore, we are reaching out to communities and community leaders across London to reassure them. Our strength as a city depends on our ability to stand together in such terrible times.

"If anyone sees anything suspicious or anything that causes concern, please do contact us, don't hesitate.

"My thoughts are with all those who have been affected by today's attack. And as a service we have lost one of our own as he acted to protect the public and his colleagues.

"This is a day we planned for but we hoped would never happen.

"Sadly it's now a reality.

"We will continue to do all we can to protect the people of London."

Mr Rowley said the officer who was attacked was one of the armed officers who protect Parliament, and detectives currently believe it was a lone attacker.

He said: "We are satisfied at this stage that it looks like there was only one attacker, but it would be foolish to be overconfident so early on.

"For a precautionary basis are locking down the area and doing all the necessary searches so we can be completely confident that everyone is safe."

Mr Rowley said that, apart from the police officer and his attacker, it was his understanding that the two other victims died on the bridge.