Eyewitnesses said he was shot by police as he approached a second officer clutching his knife.
In dramatic images a policeman is seen pointing a gun at a man lying on the floor as emergency services rush to save a victim nearby.
A knife is seen on the ground near the foot of the armed policeman.
Police officers surround the man as he is strip searched on the ground.
At least four people have died after a knifeman mowed down pedestrians on Westminster Bridge then stabbed a policeman in front of Parliament before being shot by armed officers.
Eyewitnesses described scenes of terror as the attacker was shot several times as he approached a second officer within yards of the Houses of Parliament.
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.