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.
Staff inside Parliament were told to stay inside their offices.
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, as an ambulance entered the gates to New Palace Yard.
The Board of Deputies of British Jews tweeted after the incident: “Our thoughts and prayers are with those caught up in the attack on Westminster this afternoon.”
The Community Security Trust tweeted their thoughts to those involved, before adding: “We are monitoring the situation and are in contact w/ Police. There is no indication that it is linked to the Jewish community at this stage”
Jewish neighbourhood watch group Shomrim tweeted that they “are liaising with @metpoliceuk counter terrorism. This is being treated as an isolated incident.”
Witness Don Brind told the Press Association he heard shots being fired and saw two people apparently injured on the ground.
Mr 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”.
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.”