Wednesday afternoon: the usual dissection of Prime Minister’s Questions and then bang. There was no time to hope for the best. The sound of panic rising from the streets below and, seconds later, gunshots in quick succession: three or four in total.

Two bodies lay on Parliament’s front lawn, one a police officer, the other the man who killed him.

It was 2.40pm. Two more people had been killed by a car speeding down Westminster Bridge and mounting the pavement. At least twenty more were injured, many "catastrophic".

The car had crashed into the railings of the palace. The driver then ran to the palace gates, where he stabbed a police officer several times.

Police with automatic weapons had sprinted across the grass beneath Big Ben, some uniformed, some plain clothed. Outside the entrance to Westminster Hall, thirty yards at least inside the perimeter of the palace, a man had been shot dead.

Another, a police officer, lay near him. The Conservative MP Tobias Ellwood, a former British army captain who lost his brother in the 2002 Bali bombings, gave him mouth to mouth resuscitation and CPR. Forty minutes later he would be covered over with a plastic sheet. Three hours later, he was confirmed killed.

The target was no accident: the bricks and mortar of the heart of British democracy. But the victims wandering over Westminster Bridge could have been anyone.

"We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out," a man called Rick Longley said.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw.”

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

The press offices at Westminster run all along the top floor of the palace, from Big Ben then around to Westminster Hall, directly overlooking what was clearly an incident of immense seriousness. Journalists had rushed to the windows, predictably pulling down the anti bomb-blast curtains for a better look.

A large black car was smashed against the high black railings opposite Westminster tube station. From office windows over Westminster Bridge, people had watched it mount the pavement and leave a trail of casualties in its wake, before twisting through ninety degrees and crashing into the barriers of the parliamentary estate. Its driver had escaped, running past tourists on the packed pavement, a knife in his hand, and round to the front gate where police officers wait to let ministers and their ministerial cars in and out.

A second witness, Tawhid Tanim, told The Independent he heard three shots - "bang, bang, bang" - some 10 or 15 metres from the Cafe Nero coffee bar where he was waiting for friends.

He said: "It was so loud. People were running like crazy, I couldn't see it properly. I started running.”

Police officers told the crowds to "just keep running," he said.

Westminster attack: Key moments caught on camera

In the courtyard behind the cloisters, the Prime Minister was bundled into her silver jaguar and driven to Number 10 Downing Street. The Palace of Westminster is like any large office with thousands of staff. All life is contained within. Chefs, hairdressers, maintenance men, librarians and yes, politicians, heard the air ambulance land, and watched out of windows as attempts were made to revive the two men, one, quite evidently a killer.

Two paramedic teams worked with equal diligence on both men.

Almost unnoticed on the cobbled stones around the green, where classes of schoolchildren stop every day for photographs, lay a large knife.

The division bell had rung at the same time as the gunshots, calling MPs to the commons chamber for a vote on pension reform. Hundreds of them were held there all afternoon. News of the possibility of mass casualties on the bridge outside swept through the palace like a wave.

Richard Tice, a leading figure on the Brexit campaign, said he had seen “ten bodies” on the bridge.

Outside areas were suddenly off limits. Every door had an officer outside it, sending people back where they had come from. Parliamentary passes would no longer open any door, any gate.

The scene where the police officer died outside Parliament (PA)

A second wave of panic swept the building twenty minutes later, spread by what turned out to be the loud shouts of Counter Terrorist Specialist Firearms Officers in their blue/grey uniforms, moving through every courtyard and corridor of the vast building. Armed police are part of the furniture in the Palace of Westminster. The CTSFO are not.

At six o’ clock, police confirmed four people had been killed, the killer and the police officer included. A further twenty injured.

The coastguard confirmed a woman had been pulled alive from the Thames. Within minutes it clarified she had suffered serious injuries.

A spokesman for the Port of London Authority said: "A female member of the public was recovered from the water near Westminster Bridge. She is alive but undergoing urgent medical treatment on a nearby pier. We believe she fell from the bridge."

He said the river has been closed from Vauxhall to Embankment "as part of the security response".

Below the stairs at Westminster Hall, where members of the Royal Family stand guard over the coffin when one of their number is laid in state, visitors, tourists, constituents who had arrived for meetings with their MPs were kept in offices unable to leave. In the grand hall where global statesman occasionally address both Houses of Parliament, hundreds of people were held - all potential witnesses.

One man had arrived for a job interview at the Policy Research Unit. “I was meant to meet them in Central lobby,” he said. “I never got that far.”

An amateur boxing team, the British Lionhearts, had been holding their pre fight press conference in one of Westminster’s many rooms before their bout at York Hall on Thursday. “We’ve got to eat,” one said. “We’ve a fight tomorrow.”

A couple of civil servants, called in for meetings with ministers, never made it that far either.

Reaction from the scene for terrorist attack at Westminster

In a joint statement the Commons Speaker John Bercow and Lord Fowler, the Lord Speaker, said: "On behalf of Members of both Houses of Parliament, we wish to offer our thoughts to all those affected and their families. We would also like to express our gratitude to the police and all emergency services."

The heart of power is always a heartbeat away from lockdown. The cops, the sniffer dogs, the giant barriers are not here as a precaution.

Police, politicians, the security services have warned over many years the inevitability of a London terror attack. But nothing is truly inevitable until the moment it happens.