The two airports are the focus of a huge security operation in London, set in motion earlier this week in response to a threat that Prime Minister Tony Blair has refused to detail.

The Guardian newspaper reported that the alert was sparked by "high-quality" intelligence that Islamic extremists had smuggled SAM-7 anti-aircraft missiles into Britain from Europe. Police refused to comment on the report, which said the intelligence was the most specific information about an imminent threat since the Sept. 11 attacks.

Gatwick's North Terminal was closed for hours and the terminal's flights were suspended after police found the grenade in the baggage of a 37-year-old Venezuelan after he arrived on a British Airways flight from Caracas, Venezuela.

At Heathrow, police said two men were arrested in nearby Hounslow as a precautionary measure and were not believed to be linked to the threat to the airport. They were being held at a police station in west London but police did not say why they were detained.

Britain has deployed hundreds of soldiers at Heathrow, Europe's busiest airport, over the last three days after police warned that the al-Qaida network might try attacking London during this week's Muslim festival of Eid al-Adha.

Some critics have suggested the government has overstated the terrorist danger to bolster support for a war with Iraq.

Home Secretary David Blunkett said the arrests showed officials were not exaggerating the risk to the British public.

"First, it reinforces that we really do have a problem, as people have been saying that it doesn't exist," Blunkett said. "Second, it means that our security services are on the ball. Third, over the next few days we need to follow these leads through."

In Parliament before the arrests, lawmakers pressed the government to say more about the threat that led to the increased security, but Blair refused.

"We cannot start disclosing details of everything we know or may know," he told a news conference. "But it's important we take every precaution we can in order to keep people safe."

Police said the Venezuelan man, arrested under the Terrorism Act, arrived at Gatwick on British Airways flight 2048, which originated in Caracas and stopped in Bogota, Colombia, and the Caribbean island of Barbados en route to London.

It is believed the suspect boarded in Caracas. British Airways has canceled Friday's scheduled flight to Caracas while an investigation is carried out into security checks there.

The man was taken to a central London police station for questioning by anti-terrorism officers.

A Home Office spokesman said police did not yet know what the man's intentions were and urged the public not to jump to conclusions.

The Ministry of Defense said a Nimrod surveillance plane was flying over the London area, but added that fighter jets had not been deployed. Police have boosted security in the center of the capital and at several other airports including Stansted, London's third airport, and Manchester International Airport in northern England.

(China Daily February 14, 2003)