By Tom Geoghegan BBC News Online, at Heathrow

As Scotland Yard declared heightened security in London, BBC News Online went to Heathrow, where 450 troops were deployed as part of the anti-terrorist operation. Passengers arriving at Heathrow expecting hundreds of heavily armed soldiers to be charging through the airport would have been relieved.

Initially, the only evidence of increased security was a uniformed policeman buying a newspaper at WH Smith.

After an hour searching three terminals on Tuesday morning, there was still no glimpse of anything khaki, just a handful of police officers.

Passengers carried on as normal

Army to guard London: click here

Staff were overheard asking each other if they had seen any soldiers, following the headlines in the morning, and some passengers were equally puzzled.

New Zealander Gary Arnott, 51, waiting for a flight to South Korea, told BBC News Online: "I saw on the news that 450 soldiers were being drafted in.

"But I haven't seen anything and I've been sitting here for 25 minutes."

My only sight of the Army was outside Terminal Three arrivals, where four armoured vehicles were parked.

Lieutenant Richard Moger, 25, of the Household Cavalry, spoke to BBC News Online from the top of his tank-like vehicle.

Seeing people with guns actually makes me feel insecure Troy Cartwright

He said: "We're just here to assist the police in their daily activities - we'll help them if they need anything.

"I don't think the public have paid that much attention.

"The reaction tends to be a glance, but most people just carry on their way."

Maureen Ellis, 69, from Essex, was so reassured by the sight of the troops that she stopped to have a chat.

She said: "It makes me feel much safer. That one driving the tank looks like my grandson.

"We'd seen it on the news this morning so it was no surprise to see them."

Some of the troops chatted to passers-by

The military part of the operation is probably far more discreet than many expected.

Much of the Army's activity is out of sight to passengers rather than a visible presence inside the building.

Soldiers patrolled the perimeter road to deter anyone from sneaking into the grounds of the airport.

Troy Cartwright, 21, who was preparing to fly to his native Melbourne, said the troops were better off out of sight: "Seeing people with guns actually makes me feel insecure.

"Even the police here have them - a big thing walking right past you.

"My first thought was 'My God, what's the world coming to?'"

Fascination

Claudine Dupuis, 22, on her way to Montreal, asked: "Do we need that?

"Even in the States, they don't walk in the street and airport with them."

For others, the military presence was a rare opportunity.

Wab Chowdhury, 27, busy explaining the sight to his fascinated young nephew Kamil, said: "I've lived in London all my life and I've never seen a tank this close."