IT happened in an instant; just after 230pm. Shots were heard and my colleagues and I immediately crouched down onto the floor.

My office, two floors up in the Commons press gallery, has a panoramic view of Parliament Square and below us just a few metres away is New Palace Yard, where the incident took place and where ministerial cars enter and park nearby.

As I tentatively made my way towards the window, which has anti-bomb blast net curtains, to see what had happened, shouts could be heard. I saw people rushing around and two bodies lying on the ground.

Armed police rushed over to attend the injured while others pointed their guns towards the iron railings that screen the pavement between the Commons and Bridge Street that runs onto Westminster Bridge.

Looking down from my vantage point I could see one body nearer our office window and directly outside the entrance to the historic Westminster Hall and one a few yards further away towards the main gated Commons entrance, known as Carriage Gates.

Soon, it appeared that not only armed police were at the scene but paramedics, plying bandages to the two injured people. It was a scene of urgency as those tending the injured could clearly be seen vigorously applying CPR, trying to rescusitate them.

As this went on, more police cars arrived on the scene and Parliament Square was swiftly cordoned off with vehicles and tape. Armed police officers could be seen running around the perimeter of the area, no doubt trying to see if the knife-wielding assailant had an accomplice or accomplices.

One colleagues muttered nervously: “This is just like a Hollywood movie.”

Some 15 minutes after the incident with the two injured still being attended to, an air ambulance was seen landing noisily in Parliament Square. Two ambulances arrived at the same time.

More medical staff were now on the scene. The person nearer my window had a good dozen people around him and it seemed as if there was more urgency around this person than the other one. But both were receiving CPR.

Then more activity as vehicles pulled up outside Carriage Gates and a line of armed and helmeted anti-terror police arrived, wearing bullet-proof vests. They congregated outside Westminster Hall for a conference with other officers and then disappeared in different directions.

By now 30 minutes had elapsed. Police officers with sniffer dogs could be seen running around the perimeter of the Commons precinct. A person in a white forensic suit was spotted walking quickly towards Westminster Bridge, where it appeared others had been injured.

By 3.15pm one ambulance was leaving with the person injured near Carriage Gates while it seemed hope had been lost for the person lying on the cobbled yard nearest my office window.

A sheet now covered the dead body, which was surrounded by debris, including dozens of bandage pads, a collection of bags and a stretcher. The ambulance was still there with its back door open but no one was inside. The scene, which had been a hive of activity just minutes ago, was now eerily still. Police officers were still milling around. One could be seen nonchalantly drinking a mug of tea.

Staff at Westminster have always been alerted to the prospect of a terror attack. Given the iconic nature of the building, it is clear that someone at some time might decide to target it. That some time was today.