For the second day, Britain remains on an extremely high terrorist alert. Police and soldiers are patrolling the country's main airports and roads along nearby flight paths, as well as other key potential economic and symbolic targets.
In the capital and elsewhere in the country, troops in armored vehicles and anti-terrorist police with automatic weapons are visibly out in force.
The huge security presence can be seen particularly around London's Heathrow Airport where 1,500 security personnel are patrolling the area.
The chairman of the ruling Labor party, John Reid, says alarming intelligence about a spectacular attack forced the government to act quickly.
"This is about a threat of the nature that massacred thousands of people in New York," he said.
Officials have not been specific about the threat, but press reports indicate there is concern about a surface-to-air missile attack on a passenger plane by al-Qaida operatives or those sympathetic to their cause. Terrorists tried to shoot down an Israeli airliner in Kenya late last year.
Given the scale of the threat, Home Secretary David Blunkett says one option was to close Heathrow, one of the world's busiest airports. But the minister says that idea was rejected and the heavy deployment was ordered instead.
"We hope we can get through the next few days without any incident," he said. "We are confident that we can and then we will be looking to stepping this down, returning to the kind of vigilance and security surveillance that has kept us safe since the 11th of September."
In addition to the strengthened security presence at Heathrow, similar smaller-scale operations are being carried out at other British airports.
The timing of a possible attack is being linked to the end of the Muslim religious festival, Eid al-Adha.