London police say five people, including the attacker, were killed after a car ploughed into pedestrians and a man went on a stabbing spree before being shot dead close to the British parliament.
Around 40 others were wounded in Wednesday’s attack that caused chaos in one of the city’s busiest locations.
London Metropolitan Police Commissioner Mark Rowley said investigators suspect the attacker had been “inspired by international terrorism”.
“Islamist-related terrorism is our assumption,” Rowley told reporters.
He said police believed they knew the identity of the attacker but he declined to provide details.
Rowley said the violence started when a car was driven over Westminster Bridge, hitting and wounding a number of civilians and three police officers.
The car then crashed into railings just outside parliament where a man armed with a knife continued the attack and tried to enter the building.
The assailant was shot dead after he stabbed a policeman to death. The other three victims were among those hit by the car.
Many people were lying on the ground – some bleeding heavily and apparently unconscious – after the attack, which Prime Minister Theresa May condemned as “sick and depraved”.
“The location of this attack was no accident,” she said in a statement outside her Downing Street office late in the evening.
“The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech.”
Some of the wounds suffered by the victims were described as “catastrophic”. One woman was pulled out alive from the River Thames with serious injuries by port authorities.
Witnesses initially reported multiple attacks near parliament.
“We saw a black vehicle at full speed and it ran down a number of people. I could see people flying all around,” tourist Babi Nagy told Al Jazeera. “Immediately it came to mind this was a terrorist attack.”
Polish politician and journalist Radoslaw Sikorski posted a video on Twitter of the aftermath on the bridge, showing several wounded people lying on the ground.