A heightened state of alert around key buildings and bases has fuelled fears that Britain might be subject to attack by the international terrorists currently under British and American fire in Afghanistan.
Scotland Yard maintained that high-profile policing in the capital, and a tightened "ring of steel" around the City of London, was primarily intended to reassure the public, and repeated the insistence of the Prime Minister, Mr Blair, that security and intelligence chiefs had no evidence of any current explicit threat to Britain.
However government buildings were on level 2 "amber" alert - just one down from "red" when an attack is believed imminent.
As anti-war demonstrators prepared to converge on Trafalgar Square and Whitehall in the first of a series of demonstrations across the country, specialist police surveillance vehicles equipped with hidden video equipment reinforced security precautions around possible prestige targets.
These could Downing Street, Buckingham Palace, the Houses of Parliament at Westminster, the United States Embassy and Scotland Yard itself.
Police have confirmed the beginning of this long-term monitoring operation in these and other unspecified locations, coupled with the secondment of an additional 150 officers to the anti-terrorist squad and the opening of a round-the-clock 24-hour specialist control unit at Scotland Yard to monitor security in the capital.
Security for members of the royal family was tightened as Queen Elizabeth returned to London from her summer holiday in Balmoral, amid some speculation that she might broadcast to the country.
Armed police were as usual visible at London's Heathrow Airport and other airports across Britain, as police reinforced the ring of steel around the capital's financial quarter which appeared to have been relaxed in response to the Provisional IRA's ceasefire.
Ministry of Defence police likewise maintained very visible security levels around military and security establishments across Britain.
MI5, the security service responsible for countering domestic terrorism, has advised British and American companies about measures to be taken, and has similarly advised the public utilities about security arrangements at nuclear plants and gas, water and electrical installations.
With an extra 1,500 police officers already on London's streets, a massive operation is also under way to protect London's Muslim communities from possible attack by right-wing extremists.
Police in each London borough have identified potentially vulnerable targets, while contingency plans are in place for possible public disorder.
Special Branch officers are monitoring arrivals and departures at airports, ports and rail stations while hundreds of officers are engaged in the continuing effort to track down possible suspects concerned with the September 11th attacks.