It was the second airport incident in two days. Thursday, officials closed part of London Gatwick Airport after finding a grenade in the luggage of a passenger arriving from Venezuela.
With security at Heathrow, Europe's busiest airport, at a high level because of a terrorist threat, police discovered the bag in Terminal 2, which mostly serves European destinations, at around 11:20 a.m. and emptied the building.
Officers checked and cleared the bag and allowed passengers to return about 90 minutes later.
Earlier Friday, police said they had arrested four terror suspects near Heathrow, bringing the total number of people detained during the four-day security operation to seven.
Thames Valley Police arrested four suspects in Langley, five miles northwest of the airport, on Thursday afternoon. Police did not say whether the arrests were connected to the terror threat officials say is facing London, or whether any weapons were seized.
Police were also questioning a 37-year-old Venezuelan man arrested Thursday at Gatwick airport for carrying a suspected live hand grenade in his luggage. He arrived on a flight from Caracas on Thursday afternoon, and his arrest led officers to shut the airport's North Terminal for several hours.
Officers were also questioning two men detained Thursday in Hounslow, near Heathrow. Police did not believe they were linked to the threat to the airport, which is Europe's busiest.
Police have warned that Osama bin Laden's al Qaeda network might try attacking London during this week's Muslim festival of Eid al-Adha. The warning prompted visibly higher security at major airports and in the center of the capital.
The measures included positioning some 450 troops and light tanks around Heathrow. Home Secretary David Blunkett said he even considered closing the airport, but thought the move would be costly economically and symbolically.
"We're taking every measure necessary to keep London safe and the evidence of our activity is out there on the streets," the Metropolitan Police's Deputy Assistant Commissioner, Andy Trotter, told the British Broadcasting Corp. on Friday.
Trotter said tighter security would continue for "as long as is necessary."
Britain is America's closest ally in the possible war against Iraq. It is also home to a significant Muslim population, a small number of whom are allied to radical groups.
Shoe bomber Richard Reid and alleged "20th hijacker" Zacarias Moussaoui both worshiped at North London's Finsbury Park mosque.
Britons are more accustomed to anti-terror measures than Americans, thanks to a number of bombing campaigns by Irish republicans going back several decades. One subtle example is the absence of trash canisters, which could conceal bombs, from downtown London.
Some in the United States have cited Britain's security infrastructure as a model for American efforts at reform following pre-Sept. 11 intelligence failures. Britain has a strict separation of responsibility between MI-5, handling domestic activities but not general law enforcement, and MI-6, the foreign intelligence office.