At least two people have so far confirmed to have died in the incident.
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.