LONDON, JUNE 10 -- A bomb exploded Saturday night in a military barracks rented for a birthday party, injuring 17 people, and police said today that the attack had "all the hallmarks of the IRA."

The explosion came just before midnight at the Honorable Artillery Company barracks, near London's financial district. All of those injured were civilians.

There was no immediate claim of responsibility for the bombing, but police said it was similar to attacks by the Irish Republican Army, which targets British soldiers and military installations.

"This should invoke abject contempt for the IRA throughout the whole of the United Kingdom and the Republic of Ireland," Prime Minister Margaret Thatcher told reporters on a flight back from a four-day visit to the Soviet Union.

Scotland Yard said the blast was caused by a "bomb of some sort." Detective Chief Superintendent Derek Willison, deputy head of the police force's anti-terrorist squad, said the device had been placed on the roof of the building.

"There has been no claim, so it is hard to say, but it has all the hallmarks of the IRA," Willison said.

The explosion ripped through the ceiling of the third-floor bar below, showering shattered glass and debris on the 30 guests attending the black-tie party.

Twelve women and five men were treated for cuts and shock at nearby St. Bartholomew's Hospital and released. Hospital spokesman David Dalton said two women were kept overnight for observation of possible concussions.

Police said the only connection between the party and the military was that Capt. David Venn, father of Mark Venn who was hosting his 21st birthday party, is a member of the army reserves. Capt. Venn said the party had been held in another part of the building and guests decided to have a nightcap at the bar.

"I was standing by the bar getting some drinks when there was an explosion and a flash of light. I looked at the floor -- pieces of glass, bits of plaster," he said.

The bar, on most Saturday nights, is full of reserve soldiers.