There was a major security alert at the Palace of Westminster after a man apparently carrying a knife charged through the gates into the front yard of the parliamentary compound.
Amid shouts and screams, sounds similar to gunfire rang out.
Two people were seen to be lying within Old Palace Yard, immediately outside Westminster Hall.
The sitting in the House of Commons was suspended while police officers sealed off the area around the incident.
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.
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.
An ambulance entered the gates to New Palace Yard.
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.
Radosław Sikorski, a senior fellow at CES Harvard tweeted that a car has hit five people.
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.”
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.”
One onlooker tweeted: “Just saw Parliamentary security men shoot a man who had attacked a policeman. Impressive reaction times by police.”
A spokesperson for the Metropolitan Police said: “We were called at approx 2:40pm to reports of an incident at Westminster Bridge. Being treated as a firearms incident – police on scene.”
The PM is said to be okay.