The attack in London appears to have unfolded over several locations, including on Westminster Bridge, where witnesses say a car crashed into pedestrians.
It all climaxed with loud bangs heard inside parliament, and, shortly afterward, two more bodies were seen lying on the ground in a courtyard just outside.
Along with those killed, at least 40 more people were injured on the bridge, where people lay on the ground, some bleeding heavily and one under a bus.
London's acting deputy police commissioner, Mark Rowley, quickly announced English police had declared the situation a terrorist attack.
"Parliament has been locked down, and the Met responded in line with our plans for a marauding terrorist attack. That response included uniformed and specialist firearms officers. We now, of course, have an ongoing operation, and, whilst we currently believe there was only one attacker, I am sure the public will understand us taking every precaution in locking down and searching the area as thoroughly and exhaustively as possible."
Witness Rob Lyon has described what he saw on the bridge.
"I heard a big sort of crunch - sounded like a car crash, but then almost that crunch of a wheel on a kerb. Then I saw the vehicle mount the kerb, and it was coming at us so quickly. I saw some people be hit in front of me. James, who I was with, he sort of shouted, 'Get out the way,' I think it happened so quickly. And I jumped to the left into the road. I think one of the guys had been hit. And, fortunately, it went past me. And then, (I) looked around me in shock, because I could see bodies and people and it was all a real shock."
Another witness says he watched as the car zigzagged through a street near Britain's parliament.
"Up to now, inside, outside, I'm feeling sick, and I don't know what's happening. I don't know who's that person, is it a sick person or a terrorist, I don't know exactly who is that person. At the moment, though, eyewitnessing it is not something I'd wish for anybody."
Another witness says police moved swiftly to contain the situation.
"Police were ushering people out of the Tube station very quickly. I was ushered onto Westminster Bridge. But then I looked across the west pavement, and there were bodies, people, lying on the bridge, being tended to. So then I moved up into the middle of the bridge, looked further down the south side of the bridge, it was clear there were also bodies, people, lying on the floor on the western pavement."
Britain's House of Commons, in session at the time, was suspended, and MPs were asked to stay inside.
The attacker was inside the gates of parliament when he was shot.
British prime minister Theresa May was immediately ushered to safety and kept updated on the situation.
British interior minister Amber Rudd says the government's top priority now is people's security.
"I urge everyone to remain calm, but to be vigilant. And if they see anything they are concerned about, they should report it to the police. We have the best police, the best security services, in the world, and we must make sure that we let them get on with doing their job."
The incident has happened on the first anniversary of the attacks that killed 32 people in Brussels.
It is the deadliest attack in London since four British militants killed 52 commuters and themselves in suicide bombings on the city's transport system in 2005.
