A policeman has been stabbed and his suspected attacker shot by officers in a major security incident at the Houses of Parliament this afternoon.

Around three shots rang out outside the Palace of Westminster after a man ran through the gates into the front yard of the parliamentary compound apparently waving a knife.

Two people were seen being treated on the ground in New Palace Yard amid shouts and screams.

As Parliament was on lockdown Radoslaw Sikorski posted a video to Twitter purporting to show people lying injured in the road on nearby Westminster Bridge.

Sikorski, a senior fellow at Harvard Centre for European Studies, said he saw at least five people lying on the ground after being "mown down" by a car.

"I heard what I thought what I thought was just a collision and then I looked through the window of the taxi and someone down, obviously in great distress," he told the BBC.

"Then I saw a second person down, and I started filming, then I saw three more people down, one of them bleeding profusely."

A line of red buses and several ambulances could be seen on Westminster Bridge.

Alex Wafer, 47, who was cycling nearby and saw the aftermath, said: "At the end of the embankment it appears as though there is an ambulance.

"It looks as if they are treating somebody.

"And just below, it looks like there may be another paramedic treating someone else."

An air ambulance landed in Parliament Square and a regular ambulance came in through the front gates as medics rushed to help the injured people.

Armed police cleared the area around the incident and Parliament Square was closed to traffic.

A Downing Street source confirmed that Prime Minister Theresa May was "OK".

May was seen being ushered into a silver Jaguar car as what sounded like gunfire rang out at Parliament during the incident at around 2.45pm.

As the sitting in the House of Commons was suspended, Commons Leader David Lidington told MPs: "What I am able to say to the House is there has been a serious incident within the estate.

"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.

"An air ambulance is currently attending the scene to remove the casualties.

"There are also reports of further violent incidents in the vicinity of the Palace of Westminster but I hope colleagues on all sides will appreciate that it'd be wrong of me to go into further details until we have confirmation from the police and from the House security authorities about what is going on."

Dennis Burns, who was just entering Parliament for a meeting when the security alert happened, told the Press Association: "As I was coming through the doors at Portcullis House, a policeman grabbed someone who was coming in and threw him out.

"As we were coming though the glass doors I was told by one security guard to get out while another one told me to get in.

"As I walked in I heard a security guard get a radio message saying 'a policeman has been stabbed'.

"Then I walked in as police officers and security start rushing out of the front doors on to the street.

"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street. The first stream was around 30 people and the second stream was 70 people.

"It looked like they were running for their lives."

Passerby Radosław Sikorski, a Polish politician, spoke to the BBC.

He said: "I didn’t see a car but I heard what I thought was a collision, I looked through the window of the taxi and I saw someone down obviously in great distress."

He then saw a further four people down on the ground, “one bleeding profusely”, he said.

The SNP MP Martin Docherty tweeted: "Away from the estate at the moment. Extremely worrying for Police Officers and all @HouseofCommons Staff"

A Downing Street source declined to say where Theresa May had been during the attack, but said she had not been involved.

Minutes after the incident, an emergency services helicopter landed in Parliament Square, as sirens were heard outside.

Air ambulance medics came from the helicopter to assist the casualties.

Two people lay on the ground in the yard. One of them appeared to have had clothes removed as emergency workers attempted to resuscitate the pair.

Parliament Square was closed to traffic.

Witness Don Brind told the Press Association he heard shots being fired and saw two people apparently injured on the ground.

Brind, a researcher for MPs, said: "I heard some shouting and saw some running out of the corner of my eye and then a short time after that there was a shot. I looked and I saw a civilian on the ground, with somebody standing over him with what I assumed to be a gun.

"Then I looked and about 10 yards away, there was a yellow jacketed person on the ground, who appeared to be alive and talking."

He said he assumed the person in the high-visibility jacket was a police officer.

Pictures emerged of a car having crashed into the railings of Parliament at the end of Westminster Bridge Press Association reporter Laura Harding, who was in Westminster at the time of the incident, said: "Everyone has been evacuated into Central Lobby, including a group of schoolchildren and kitchen staff.

"Around 15 schoolchildren aged around 10, with armed police coming through the lobby now.

"The children are really calm, the teachers are comforting them.

"Everyone is standing around on their phones.

"There are also a bunch of young people from the Hammersmith Boxing Club in their tracksuits and the British Lionhearts boxing group."

The Met Police has tweeted: "We were called at approx 2.40pm to reports of an incident at #Westminster Bridge. Being treated as a firearms incident - police on scene".