Whitehall drama unfolds before tourists

This knife was retrieved by forensics officers in Whitehall

Armed police swooped in Whitehall on Thursday afternoon to arrest a man on suspicion of terrorism offences, and it was near the aftermath of that scene where we all stood now.

One of London's key roads was closed, but others in the area were open and teeming with traffic. This meant stern-looking policemen stood around, scolding press and public alike as they directed pedestrians.

"Oi! It'll be hazardous to your health if you get squashed by a lorry," barked one officer, as he motioned with some exasperation for people to get out of the way of traffic.

This man was detained by firearms officers in Whitehall

People stood on the pavement, beneath Winston Churchill's statue, craning their necks and watching the every movement of two figures across the street, so well covered by their loose-fitting crime scene suits that only their eyes were visible.

One press cameraman finally broke cover and ran across the road, when the passing cars were on a breather, to the edge of the fluttering police cordon tape. He was followed by plenty of others, photographers and tourists.

Together we watched the painstaking routine of the forensics officers. Measurement markers indicating scale were put around objects on the ground.

Some of those objects looked like knives, others scraps of paper, all of them photographed from different angles before being put into a box or a bag.

This bag was examined before being taken away

Each time this happened, a cricket-like chorus burst into life from the photographers training their lenses towards the evidence gathering.

This was repeated again and again, officers removing their green plastic gloves for a new pair each time they handled an object.

A brown rucksack, discarded on the ground, was also examined before it too was transferred into a large paper bag.

Police held a cordon around Whitehall

The tourists got bored, frozen in the biting cold around Westminster, and some went while others came to see what was going on.

Journalists did pieces to camera in front of the scene, but had to raise their voices over the incessant noise of a helicopter hovering overhead.

All this happened yards from Westminster Bridge, the scene of a terror attack weeks ago.

But all around Parliament Square, tourists continued to take selfies and pictures of Big Ben and the Houses of Parliament. And life went on.