A woman has died on Westminster Bridge in central London in what police are treating as a terrorist incident.
A police officer was also stabbed in the nearby Houses of Parliament by an attacker, who was shot dead by police.
The attacker struck several pedestrians as he drove a car across the bridge, before crashing it into railings.
A doctor at St Thomas' Hospital said a number of other people were hurt - some with "catastrophic" injuries.
A Downing Street source said Prime Minister Theresa May was safe.
Prime Minister Theresa May was seen being ushered into a silver Jaguar car as what sounded like gunfire rang out at Parliament during the incident.
MPs said they had heard three or four gunshots and staff inside Parliament were told to stay inside their offices.
Commons Leader David Lidington told MPs the "alleged assailant was shot by armed police".