Police said Osama bin Laden's network might time attacks to coincide with the end of this week's Muslim Eid al-Adha holiday. The religious festival takes place Tuesday in Saudi Arabia, but most Muslim communities continue celebrations for several more days.
"From time to time, it is necessary to raise levels of security activity. We think it is prudent to do so now," police said in a statement.
The Ministry of Defense said some 450 troops were being deployed in response to the threat, but wouldn't say if they would all be stationed at Heathrow, Britain's busiest airport.
Troops from the Grenadier Guards regiment and the Household Cavalry had been assigned to security duties using Scimitar light tanks, the ministry said. The units, which guard Buckingham Palace, home of Queen Elizabeth II, are highly trained combat troops.
"This morning at about 6 a.m. the police took a number of measures to strengthen security in London. The most visible element of this arrangement is at Heathrow," airport officials said in a statement.
A police spokesman said the army is not normally used to guard Heathrow, west of London. He wouldn't say whether the army was being deployed at other sites, or how long the operation would last.
Britain is Washington's main ally in its confrontation with Iraq and the security warning for Heathrow comes after the Bush administration on Friday raised the national terror alert from yellow to orange.
"The strengthened security, which is likely to be most visible to the public at Heathrow Airport, relates to a potential threat to the capital," the police statement said. "The heightened arrangements include the use of military personnel in support of the Metropolitan Police."
Police said there was a "possibility … that al Qaeda and associated networks" would use the end of the Eid al-Adha festival to mount attacks.
On Tuesday, a radical Islamic activist warned people to stay away from government buildings and financial institutions, saying radical Muslims were prepared to act as suicide bombers.
Sheikh Omar Bakri Mohammed, a leader of the small Islamic group al-Muhajiroun, said Tuesday that al Qaeda supporters in Britain might carry out attacks.
"I would warn people to take precautions. Do not go into government buildings do not be in any financial institutions, keep away from these locations," he told British Broadcasting Corp. radio.
Al-Muhajiroun is part of a radical Islamic community in Britain that includes the mosque where shoe bomber Richard Reid and suspected 20th Sept. 11 hijacker Zacarias Moussaoui once worshipped.
Police in Britain have arrested several terrorist suspects since the Jan. 5 discovery of the deadly poison ricin in a north London apartment. Reports have indicated a suspicion that the ricin was going to be used to poison food at a nearby military installation.
In a report to the Security Council last week, Secretary of State Colin Powell linked the apparent ricin plot to alleged al Qaeda operative Abu Musab Zarqawi, whom the Bush administration claims Saddam Hussein has given refuge.
Attorney General John Ashcroft mentioned the ricin plot in his explanation of the increase in the U.S. terror alert system from "elevated" to "high."
Britons are used to seeing more security than Americans — bombing campaigns by the Irish Republican Army led to the removal from downtown London of most trash canisters.