In another blow of a widening IRA terror war, a bomb blast today tore through an open-air stage where a British Army band was preparing to give a concert for tourists in the Belgian capital's historic Grand Place. Eleven spectators and four bandsmen were slightly injured, police said.

Brussels Mayor Pierre van Halteren said the Irish Republican Army claimed responsibility for the bombing in a telephone call to city hall. Irish guerrillas have attacked other British targets in Belgium and the Netherlands in recent months.

The bombing in the Belgian capital came just a day after IRA bombs killed Britain's Earl Mountbatten of Burma and three others in the Irish Republic, and 18 British soldiers in Northern Ireland.

The band that came to Brussels was the Duke of of Edinburgh's Royal Regiment band, stationed in Ossendorf, West Germany.

It was about to begin a 1 1/2-hour concert in the broad plaza, a major tourist site surrounded by centruries-old buildings. But by chance, only six of the 24 members of the band were on stage when the blast ripped apart the floor about 3 p.m.

"We were lucky because we arrived late," said band member Jack Hardy. "We were held up by a traffic jam getting into Brussels."

The others had stepped off to change into their red dress uniforms after setting up music stands and instruments.

Even before the IRA phone call was reported, Earl Nicoll, military attache at the British Embassy here, said: "I'd guess it is either the IRA or people sympathetic to their aims. It is clearly a manifestation they wanted to hit the band, not any Belgians."

The temporary stage has been used for daily concerts to mark Brussels' 1,000th birthday this year. A police spokesman said the explosives were under the stage floor in the back, on the side away from the square. At the time, a band member said, "a few hundred people" -- most of them tourists -- were in the square.

The blast ripped a gaping hole in the 90-by-30-foot stage floor, ripped open the back wall and tore a big crack in the ceiling. It shattered windows in guild halls lining the square, called a major example of medieval and renaissance architecture. Nicoll said, "it was a miracle" that nobody was killed.

A spokesman for the St. Pierre Hospital said all the injured would be released tonight except for one Briton, apparently a band member, who was being kept under observation for 24 hours. The injured, who included two children, suffered from cuts and shock.

Officials estimate the blast caused between up to $167,000 in damage.