Confirmation has been announced that a woman has died and a number of others have been hurt - including some with "catastrophic" injuries - in the incident on Westminster Bridge, a junior doctor at St Thomas' Hospital has said.
Paramedics fought to save his life and that of his victim on the floor of the cobbled courtyard in front of Parliament with Foreign Office minister Tobias Ellwood among those who rushed to help.
Colleen Anderson, a junior doctor at St Thomas’ hospital, said a woman was killed as the attacker drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings then running through the gates of the Palace of Westminster.
She also said she treated a police officer in his 30s with a head injury who had been take to King’s College hospital.
She said: “I confirmed one fatality. A woman. She was under the wheel of a bus.
“She died. Confirmed her death at the scene.”
At least one of the people being treated in the palace courtyard was wheeled away on a stretcher with their face covered.
The Metropolitan Police are treating today's attack on the Houses of Parliament as a terrorist incident 'until we know otherwise'.
A police officer is thought to have been stabbed by an assailant who was then shot by armed units on the parliamentary estate.
A car is also believed to have been driven into a crowd of pedestrians on Westminster Bridge.
MPs were on lockdown in the Commons and the entire complex is now in the process of being evacuated.
Prime minister Theresa May has also been evacuated to a safe location.
Police have appealed for members of the public who may have filmed the incident to share their footage with investigation teams. Visit http://ukpoliceimageappeal.co.uk/CSIPS_Public/upload/choose to upload.
Scarborough MP and immigration minister Robert Goodwill said he was coming out of the voting chambers when the lockdown was put in place.
He said: "I'd just finished voting and was heading to my office to await the next vote when we were put into lockdown.
"We were told there had been some kind of incident and were told Parliament had been suspended.
"We have now been locked into our offices while the police investigation is ongoing.
"My thoughts are obviously with the police officer and, from the reports we are getting, the other people who may have been caught up in this."
Harrogate and Knaresborough MP Andrew Jones is currently trapped in a room inside the building.
"This is a rapidly developing story. I was in the Palace of Westminster, I had just been casting my vote then we got got the news of this security incident, the security procedures have kicked in and we have gone into lockdown.
"I cannot tell you exactly where I am but the doors have literally been locked down and we are waiting. I have spoken with the director of security, and this is a developing story I should say, but I think the person in the car drove across Westminster Bridge, he climbed onto the roof of his car and was able to get into the palace grounds and it is believed that he has been shot.
"It needs to be clear it is believed there is not more than one person but the security services are proceeding to be safer rather than sorry. We cannot go