The United States and Britain are maintaining their state of high alert today after what both states say is a credible threat of attacks by al-Qaeda.
Anti-aircraft missiles and fighter jets have been readied to defend the US capital and troops are patrolling London's Heathrow Airport.
An RAF Nimrod is patrolling the skies above London in support of troops and police on the ground.
With the Bush administration denouncing an "unholy partnership" between Osama bin Laden's al-Qaeda network and Iraq, soldiers joined police at likely targets, with tanks and armoured cars deployed at Heathrow.
US defence officials said jeep-borne Stinger anti-aircraft missiles have been deployed around Washington this week after reported threats from sources with ties to al-Qaeda triggered an orange alert, the second-highest level.
CIA Director Mr George Tenet, speaking after a tape attributed to bin Laden urged Muslims to fight the "allies of the devil," told Congress an attack could come in the United States or on the Arabian Peninsula as early as this week.
"He's obviously raising the confidence of his people, he's obviously exhorting them to do more, whether this is a signal of impending attack or not is something we're looking at," Mr Tenet told the Senate Armed Services Committee, referring to bin Laden.
Citing security concerns, Washington offered flights home for non-essential staff at its embassies in Bahrain and Qatar, where US forces have gathered for a possible invasion of Iraq.
In London, Prime Minister Tony Blair told parliament the security measures were "necessary in order to give people the protection and security they need". Home Secretary Mr David Blunkett said the government had even considered closing Heathrow airport, among the world's busiest.
A total of 17,000 police were on counter-terrorist duty across London, and police evacuated King's Cross, one of Britain's busiest train stations, yesterday. The station reopened after a suspicious vehicle was checked.