Security measures were stepped up across the capital despite assurances from police that Britain had received "no specific threat" after the start of the Anglo-US bombing of Afghanistan.
Scotland Yard deployed 1,500 extra officers as part of Operation Calm to watch potential terrorist targets in central London and reassure the public. From the Houses of Parliament to Heathrow, from Canary Wharf to the City, large numbers of parked police vans and uniformed officers on the street bore testimony to Britain on alert.
Security measures at utilities plants, power stations and major transport hubs outside London were also ratcheted up.
A Scotland Yard spokesman said: "We are closely monitoring the events which are unfolding and vigorous plans are in place for a range of threats which may impact upon London. High-visibility policing will continue and additional security is also being introduced in areas considered potentially vulnerable. The aim is to promote calm and reassurance across the community."
There was little doubting the seriousness with which the authorities were taking their task in London yesterday.
Armed officers were discreetly stationed at sensitive locations, including the Houses of Parliament, Buckingham Palace and the American Embassy in Grosvenor Square.
Workers arriving at the City's main institutions, including the Stock Exchange and the 800ft Canary Wharf tower, were subjected to individual identity checks.
Extra patrols by police carrying Glock machine guns were ordered at Heathrow, Gatwick, Stansted, Luton and Manchester airports. At RAF Northolt, the north-west London VIP airport used by the Royal Family and government ministers, all vehicles were being checked by a bomb squad officer in front of armed guards.
Elsewhere, police surveillance vehicles were deployed around the capital and the south-east, while undercover officers were monitoring stations, ports and other transport areas.
The Home Office said it had also taken steps to ensure the security of water, electricity and gas supplies, as well as extra measures at government offices and military bases.
A Home Office spokeswoman declined to discuss security levels but a visit to any Whitehall ministry yesterday would have revealed that officialdom is on "Amber" alert – allowing random bag searches and ID checks.
Tours around Britain's 15 nuclear power stations, and the Sellafield reprocessing plant in Cumbria, were cancelled.
David Blunkett, the Home Secretary, admitted that the line between boosting public confidence with extra police and denting it by detailing every threat and counter-measure was a narrow one. "The moment we start going through the list of things we are doing, we have the reverse effect of the one we seek, namely that we start to worry people," he said.
On London's rain-sodden streets, however, there were few signs of panic. Daniel Arglow, a 43-year-old engineer from Seattle, said: "It's kind of good to come out of the Underground and find all these police milling around. Sometimes, tourists are outnumbered by the cops. I guess that means we're really at war."Reuse content