LONDON - A small explosion rattled the Hammersmith Bridge area of west London last night after telephoned warnings claimed by the Irish Republican Army.

Police said they had neutralized a multi-part explosive device after an explosion in one part of it. There were no reports of injuries or damages.

The blast came more than half an hour after one warning call to The Associated Press office. A second warning was called into The AP about 20 minutes before the explosion.

The bridge, which crosses the Thames River 4 miles west of central London, had been cleared and blocked off and a search with sniffer dogs was under way when the explosion occurred. Huge traffic jams formed on the approaches to the north side of the bridge as police worked.

Residents had reported hearing two loud blasts, but Scotland Yard would confirm only one ''small explosion'' near the south side of Hammersmith Bridge just before 11 p.m.

Police said they found an explosive device in several parts under the south side of the bridge, as the warning had indicated. It was on a footpath that runs beneath the bridge.

Yesterday was the third anniversary of a huge IRA bomb in the heart of London's financial district, which killed one person, injured 45 and caused massive damage.

The IRA has waged a 25-year campaign of violence against British rule in Northern Ireland. A cease-fire that began in September 1994 was broken in February with a bombing in east London's Docklands business district that killed two newspaper vendors.

The IRA said it ended the cease-fire because of British intransigence in attempts to bring peace to the British-ruled province.