The Houses of Parliament, with their iconic 'Big Ben' clocktower, are the defining symbol of Britain across the world. And as such, the prime target for any terror attack.
The building, itself a Royal Palace made up of both Victorian gothic and Medieval halls, is no stranger to threats against the state.
But while tourists focus on Guy Fawkes' attempted plot to blow the building up more than 400 years ago, older MPs think of more recent history whenever they walk or drive through the carriage gates entrance.
It was in 1979 that Tory MP and Margaret Thatcher aide Airey Neave was murdered by Irish nationalist terrorists as he drove from the Palace underground carpark. Until today, that was the last violent death of anyone on the Parliamentary estate.
Just two hours before the terror attack, Theresa May had been at the despatch box for the weekly Prime Minister's Question Time, and had paid tribute to former IRA man-turned peacemaker, Martin McGuinness.
May was still on the premises when the carnage unfolded on Westminster Bridge, spanning the Thames beside the House of Commons.
MPs were actually voting – on an obscure piece of pensions legislation – when the attack on democracy itself took place.
The division bell was clanging away, summoning them to the Commons chamber, as another loud noise was heard.The bang was the terrorist's car slamming into the side of the railings that serve as a boundary between Parliament and the public.
For years, those railings were almost more ornamental than secure. It was only after environmental activists once scaled Big Ben – which was actually christened Elizabeth Tower in 2012 in honour of the Monarch's 60 years on the throne – that the fence was raised and other measures put in place.
With each terrorist outrage, from 9/11 onwards, armed police have become more numerous in the Palace of Westminster.
The last time many of us in the Commons had experienced a live terror incident was on 7/7, in 2005. I remember it well, the morning after the UK had won the 2012 Olympics bid, and we had the news of bombs going off on Tubes and buses across the capital. Westminster's Underground station was closed, but Parliament escaped unscathed.
Since then, we've all been warned that a 'lone wolf' attack is the one that worries the security services most. Mumbai, Sousse, Paris, Nice, Brussels, had all seen the horrific new tactic of the Islamist extremists. Now it looked like London would be added to that grim roll call – one year to the day since the Brussels atrocities.
At 2.40pm, as they walked along the long colonnade to the Commons chamber to vote, MPs heard the bang, followed by shouting and a scream. As the public fled into Parliament Square, the suspect was seen entering through the open carriage gates, brandishing a long knife.
It was then the horrific truth dawned, and the man was seen stabbing a police officer, who staggered a few feet away. Other police not fearing for their own safety, challenged the knifeman.
The trained marksmen sprang into action, and three shots rang out in quick succession across New Palace Yard, the impeccably manicured garden in front of the 11th century Westminster Hall.Some MPs had been heading down the escalator from Portcullis House and police told them to urgently evacuate to the Underground exit that few members of the public are aware of. Some then walked over Westminster Bridge, their path "littered with bodies lying like skittles on the road", one told me.
We in the media have offices in the Press Gallery overlooking New Palace Yard and we rushed to the windows, pulled back the net curtains – specially designed to cope with bomb blast fragments - to try to see what was happening.
We could see two people lying on the ground, one an officer in a yellow hi-vis jacket, the other a man in who looked like the assailant.
In the colonnade, former Tory party chairman Grant Shapps told me how he and others heard the split-second gunfire, as officers told MPs "Get down! Now!" MPs then crawled on their hands and knees away to safety.
Shapps, a former Foreign Office minister trained to deal with emergencies in Mogadishu, knew that the thick limestone walls of the Commons were the best place to be. He headed behind the Speaker's chair and told the Deputy Speaker to stop the vote and adjourn the House.
The Prime Minister, who was in the voting lobby itself, was swiftly taken downstairs to the Speaker's Courtyard and bundled into an armoured ministerial car. The car made towards the usual exit before realising that was exactly the direction of the terror incident, and quickly reversed. She was then taken another route out and on to Downing Street.
The voting bell stopped and the horror of the scene unfolded. I saw police crowd around their fellow officer as he lay on the cobbled floor.
Foreign Office minister Tobias Ellwood, a former Army officer with the Royal Green Jackets whose own brother was killed in Al Qaeda's Bali bombing, was one of the first to offer to help. Smeared with blood from his efforts, he and others tried desperately to stabilise the stricken policeman.
As a clutch of police cars arrived, a red air ambulance helicopter descended into Parliament Square just seconds before road ambulances arrived, edging slowly through the gates. With the seconds feeling like minutes, we could see police and paramedics desperately pumping the officer's chest with CPR.
Minutes later, the assailant was also seen getting the same treatment, but the crowd around the policeman was growing larger. The concern on the faces of those gathered was palpable.
Downstairs, more armed police poured out of the underground armoury, taking up positions to train their sights on the yard. A posse of military-looking CTSFOs (Counter-Terrorism Specialist Firearms Officers) arrived through the main gates, armed and helmeted, and fanned out to secure key positions. It appeared the incident was still live, but no one knew if it was already over.
News of the awful injuries on Westminster Bridge filtered through on the TV screens as live broadcasts showed footage from helicopters above.
MPs were in lockdown in the Commons chamber, all political parties united in concern and trying to follow events on their smartphones. We in the media were also in lockdown in our offices, not allowed to enter the Press Gallery seats that we normally occupy high above.
Inside the Commons, we were all acutely aware that Parliament's rules prevent any unauthorised photos or videos on the premises. The notices on every green TV screen had a simple, stark message: "HOUSE ADJOURNED. Remain in your current location. Do not move about the building."
Down below, I could see the frantic efforts to save the policeman had stopped. He was covered with a purple blanket and a silver foil sheet. The assailant, whose trousers had been ripped open as medics tried to pump his chest, was put on a stretcher and lifted gingerly into an ambulance.
The CPR continued even as the stretcher went in the back, then the ambulance crept out of the gates. Yards away, two more grey blankets were placed carefully, respectfully, over the officer's body.
His death was all too real, and confirmed officially hours later. We don't yet know his name, but he gave his life defending democracy.
Just one week before Brexit sets the UK on a new historic chapter, the nation will unite again in its grief. And the mother of Parliaments will always remember him.
UPDATE: Police said on Wednesday night that the murdered officer was PC Keith Palmer, 48-year-old husband and father. Like many others, I've seen him around the Palace for years. We all owe him an unpayable debt.
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