Those gates have to be kept at least partially open when lawmakers are voting, to allow them to get into the building before the vote is over. They’re guarded by police whose days are generally spent giving directions and posing for photos with tourists.

Police officer Keith Palmer - who was unarmed - tried to stop the intruder. He was stabbed. A plain-clothes colleague shouted a warning to the attacker, and then opened fire with a gun. He fired several times. Journalists with offices in Westminster, could see the events unfolding outside their window.

Moments later, the wounded policeman and the assailant were both lying on the cobbles outside Parliament. One officer with a sub-machine gun kept it aimed at the downed attacker, while his colleagues tried to keep him alive.

Press Association news agency photos, believed to be of the attacker lying on an ambulance stretcher, showed a burly, bearded man wearing black clothes.

The emergency plan sets out that the thousands of people who work in the network of Parliament’s buildings should stay where they are unless they’re under direct attack. Members of Parliament were shut into the chamber of the House of Commons for hours. Others were evacuated to Westminster Abbey, another of the ancient buildings nearby.

The prime minister was whisked to a secure location - her office would only confirm that she was safe -- leaving fellow cabinet members behind her.

Outside, police officers were joined by ambulance crews and an air ambulance landed on the grass in the middle of Parliament Square. The rhythm of CPR continued where the attacker lay until shortly before his body was lifted into an ambulance. A grey sheet was placed over the body of the dead officer and a gurney - meant to take him to hospital - lay unused at his side, next to an abandoned ambulance.

Now the priority was to establish whether the attack was over. Scotland Yard, the headquarters of London’s police, is next door to Parliament, and elite firearms units, some wearing body armour over their civilian clothes, ran to the scene. Other officers were closing off the roads around Parliament, some of London’s busiest thoroughfares.

On the bridge, paramedics were rushing to help those who had been hit by the car before it crashed.

Aysha Frade, a British national whose mother is Spanish, was one of two people killed on the bridge. A man in his mid-50s also died.

Rachel Borland, the principal of DLD College — a school in Westminster a stone’s throw from Parliament — said she was a “highly regarded and loved” member of staff who worked in the school’s administration team.

Frade was 43 and had two daughters, Spain’s regional Voz de Galicia newspaper reported.

Watch: footage shows moment of London attack

A woman who plunged into the River Thames when the attacker ran into pedestrians with the SUV on the bridge was a Romanian tourist in London to celebrate her boyfriend’s birthday. She survived.

“I was walking across Westminster Bridge. I heard a wheel hit a curb, quite a large crunch noise, clearly hitting people as it came towards me,” said Rob Lyon, 34, a marketing director who was at the scene.

“I saw people being hit by a car at speed. I just went into shock mode.”

Squads of police, some carrying submachine guns, others handguns and bulletproof shields, fanned out across the Palace of Westminster as rumours flew about what was going on. There were suggestions on social media that another attacker was loose somewhere in Parliament. Officers were stationed in corridors telling people to return to their offices.

Gradually police started to evacuate the buildings, leading groups of lawmakers, staff, journalists and kitchen workers in their white chefs’ overalls into Westminster Hall, one of the oldest buildings in Europe, completed in 1099. Just metres from its doorway, the body of the downed officer still lay.

There, workers and lawmakers stood around together talking over the day’s events. Jeremy Hunt, the health secretary, spoke with reporters. A group of boxers on a visit stood around in their track suits. Ed Miliband, the former Labour Party leader, kicked his heels with everyone else.

They were all potential witnesses, police told them, as night fell.

The next day, Thursday, the Houses of Commons and Lords opened and sat as normal in a show of defiance - even allowing in members of the public to watch, as usual.

“What we’ve got to do is not give in to terrorism today,” Lindsay Hoyle, the deputy speaker of the House of Commons, told Sky News television.

“We will pay tribute to what has gone on. What we will do is continue as the House of Commons. We will not give in to terrorism, and terrorism will never ever win.”

A bouquet of flowers lay next to the security cordon outside Westminster Abbey as a tribute to the victims, while a single bloom was tucked into the knot of the police tape.

A minute’s silence was held at 9:33am across the building, marked by the ringing of the division bells that normally call lawmakers to vote.

Flags on government buildings around Westminster flew at half-mast, but many people were walking or cycling into work as usual.

Parliamentary staff were escorted into the palace through a side entrance, through the House of Lords coatroom.

There were respectful exchanges between staff, while armed police stood silently on guard.

In the cobbled yard, the bloodied rag was the only remnant of the frantic scenes where Ellwood and emergency services tried in vain to resuscitate Palmer, a 48-year-old husband and father.

A blue forensics tent stood over the spot where the assailant died, shot by a police marksman, with an ambulance parked alongside.

Another tent was installed outside the gates, where the assailant crashed his car.

Additional reporting by Agence France-Presse