In the Palace of Westminster you are never far away from security. There is a constant reminder that the Houses of Parliament and Big Ben, one of the most recognisable tourist sites in the world, the star of a million holiday selfies, is also a major terrorist target.

From the airport-style security screening to the armed police who discreetly walk the cloistered passageways, it is impossible not to be aware that you are in a building that, as a symbol of both ancient and everyday democracy, the heart of an 800-year-old parliament but also the ordinary place of work for thousands of people, is fiercely protected.

On the day of the 9/11 attacks I was working in the House of Commons, at one point diverting to Whitehall to report on the Foreign Secretary Jack Straw’s response to the atrocity, before returning to the press gallery where I worked until after midnight. It was only on 12 September 2001, sitting back at my desk overlooking New Palace Yard, that I started to think about the very real possibility that we could be next. At the time, after planes had been crashed out of the sky, the fear was of obliteration by calculated terror plots rather than a low-tech, marauding “lone actor” with a knife and a car.

Westminster attack update: How the day unfolded

The only way to process this fear is to get on with work and life – and to plan an escape route just in case. An older colleague showed me a backstairs way to the basement, down a spiral staircase, and I regularly walked through this route in my mind.

Over the years, the security barriers became more visible, the checks of passes a little more routine. After the London Underground bombings of 7/7, there were more barriers, higher fences, extra security detail. Throughout all of this, police officers like PC Keith Palmer held the line, the living embodiment of Parliament’s face to the world: open and friendly, happily posing for tourists’ photographs, yet keeping safe the heart of democracy.

When I saw this officer’s picture I instantly recognised him as someone I had seen and said "hello" and "thank you" to countless times over more than 15 years of going through those gates. He will be recognisable to so many others; he is the friendly policeman in many tourist pictures. Knowing that Palmer and his colleagues were always at the gates, standing between us and any potential attacker, meant working in Parliament felt all the more safe. Fear of terror subsided to mere calm awareness. That escape route remained, but only at the back of my mind.

For the police officers who guard us, there is, of course, no escape route.

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

There is no question of running and hiding. Palmer stepped in the way of the attacker with such an automatic heroism that the Parliament building was never breached. The terrorist did not make it beyond 20 yards inside New Palace Yard thanks to Palmer and the plain clothes armed officers who shot and killed him.

This time around, I was nowhere near Westminster on Wednesday, but I did spent a frantic 10 minutes trying to contact my partner who had texted to say he was just outside the Commons at the time.