"If police tell you to run, run." The officer's chilling instruction rang out across the quadrangle in the Commons below Big Ben.
Above the Elizabeth Clock Tower, helicopters whirred. Below, an unlikely crew of primary schoolchildren, ministers, visitors, staffers, peers and journalists listened intently to the police.
The news of the attack cascaded through the Houses of Parliament, depending on where you were.
One man, who did not want to be named and who has been working in the Palace of Westminster for 30 years, told Fairfax Media he was sitting on the terrace balcony overlooking Westminster Bridge when the car ploughed into pedestrians.
"I saw a body fall from the bridge into the water," he told me. He was still digesting what he had witnessed.
Others inside the building heard the commotion but dismissed it as one of the regular, rowdy protests that spring up outside Britain's halls of democracy and history.
For the group of foreign journalists being briefed by the Lords, the news was broken in an entirely modern, if not distant way.
A breaking news alert flashed up on one correspondent's phone and was read aloud by another.
Almost simultaneously, every reporter's phone began buzzing with urgent requests from their news desks at home.
"We get security situations all the time," one Lord scoffed.
But this was not to be. The television mounted on the wall showed the Commons, which had been voting, was suspended.
When it resumed, the Leader of the House in the Commons, David Lidington told MPs a "police officer has been stabbed" and the "alleged assailant" had been "shot by armed police".
As the Commons fell silent once more, the channel was changed to the BBC.
Parliament went into lockdown and no one was to move. For a time, we, a group of journalists, were learning more from the television about what was happening metres away from us than we were with our own eyes and ears.
Forty minutes later the order came: "Gather your things, we're leaving."
Visitors, staffers, peers alike were herded through the building as counter-terrorism police wearing helmets and face covers yelled their orders.
"Go, go, go, keep moving."
In the space of three minutes at least 10 SWAT police officers, several regular police and more police who regularly guard Parliament carrying machineguns.
"When I saw all the SWAT teams, I felt like I was in a movie; it was a surreal experience," Lailah Nesbitt-Ahmed, 27, said.
Police feared a second attacker could have penetrated the palace. In the bowels of Parliament one officer stood, semi-squatting in the doorway opening into an empty, brightly lit corridor, gun poised.
There would be no miss if a shot was required. That was certain.
Calls were made to loved ones.
"Parliament is under attack," one man said down the line - his matter-of-fact tone only emphasised the sense of danger.
All conversation stopped when we were shuffled past the crime scene.
Peering between the line of police, a body-shaped mound covered with a white material could be seen lying on the stones outside the entrance to the grand Westminster Hall, the setting of medieval banquets and contemporary state visits.
There was silence as the group took in the scene, the ambulance next to the body and the rubbish strewn on the ground around it.
But not everyone was rubbernecking.
"I couldn't look," one woman confessed.
The slain officer's colleagues, witnesses to the attack, were taken across the road to Westminster Abbey's Lady Chapel to give their statements to police.
Two hours later, just before 5pm, the thousand-strong occupants found sanctuary under the Abbey's Triforium or gallery.
It had always been planned that, if Parliament was under attack, the Abbey would be the refuge.
They would stay there for four hours waiting for enough police to arrive to take the details of every single person.
As phones ran empty and the temperature dipped further, blankets were found for those who had had to abandon their coats. But not a complaint was heard.
"No one's complaining, everyone understands that this is a crime scene and it takes a long time and if everyone has to give a statement that's absolutely what we should do," Tory MP Sarah Wollaston said after the marathon lockdown ended.
The British stoicism prevailed throughout and there was praise for police, who peers, staffers, visitors all remarked responded swiftly to the threat upon their London icons.
They applauded police after one of many mundane, housekeeping announcements, as their patriotism combined with pride, cognisant that they lost a colleague hours earlier.
