Noisy protests are a fact of life for journalists at Westminster whose offices face out on to Parliament square.
But the loud bang yesterday afternoon immediately sounded out of place. Looking out the window on to New Palace Yard, the courtyard that serves the entrance for vehicles including the Prime Minister’s motorcade, it was clear others felt the same.
The three unarmed police, guarding the gate in bright yellow jackets, moved towards the source of the noise on Westminster Bridge. It sounded as if someone had been hit by a car - but the screams and shouts drew nearer.
Suddenly, at the gate, there was a surge. The three officers recoiled, and a group of people - tourists, office workers, pedestrians - rushed forward, fleeing from danger. In the mêlée of officers and security staff, an attacker - armed with a knife - lunged at one officer. Two shots rang out, and two people fell to the pavement.
Both prone figures were immediately surrounded, and as quickly as the bid to kill was over a frantic battle to save life began. Someone shouted for a first aid kit, flung over a barrier.
Armed officers, fearing and searching for a second attacker, rushed to take up positions, their weapons raised. A security guard, racing down the corridor, shouted to stay away from the windows: “They’re targeting the building.”
Both the injured were given CPR in the moments before the paramedics arrived.
The person who stepped forward to help the ambulance crews turned out to be the foreign office minister Tobias Ellwood, a former army captain whose brother was killed in the 2002 Bali bombing. Walking away, he had blood smeared across his face from attempting to resuscitate the stricken police officer.
Within minutes, an air ambulance landed on Parliament square. One figure was lifted into an ambulance - a sheet was drawn over the other. From our vantage point, it wasn’t clear which is which. Two rings of debris mark the spots on the cobbles.
Eventually, journalists in lock down were herded into Westminster hall with hundreds of staff, visitors, MPs and peers - among them Lord Trimble, a Nobel laureate for his efforts to bring an end to terror on his doorstep. All of us may be witnesses, we were told.