Fire forced the evacuation of Buckingham Palace Saturday evening, interrupting preparations for the outdoor concert and marring the high-spirited, four-day celebration of Elizabeth’s 50 years on the throne.

The London Fire Brigade said no members of the royal family were in the palace when the fire began in a roof apartment.

Rock star Ozzy Osbourne had just finished rehearsing outside, and the British Broadcasting Corp. reported that those evacuated from the area around the palace included the musicians Phil Collins, Eric Clapton and Queen guitarist Brian May.

The concert was expected to be a high point of the national Golden Jubilee celebrations, and the palace promised it would go ahead as scheduled.

“There is absolutely no question of it not” going on, a spokesman said on customary condition of anonymity.

Also among those scheduled to perform for 12,000 guests and a live television audience are Paul McCartney, Annie Lennox and former Beach Boy Brian Wilson.

“There is an enormous crowd of extremely famous people here sitting on the lawn chatting on their mobile phones,” May told the BBC while the fire was still burning.

The blaze was reported at 6:38 p.m. (1738 GMT) and was brought under control an hour and a half later, said Colin Williamson, a Fire Brigade spokesman. Outdoor rehearsals resumed at 8:20 p.m. (1920 GMT) and workers were allowed back inside the palace at 9:40 p.m. (2040 GMT).

When the fire was at its height, smoke poured from one area of the roof as firefighters with masks, air tanks and hoses worked nearby.

News reports said it was the first time the palace had been evacuated since World War II.

One firefighter was hit by a piece of equipment and suffered cuts and bruises around his eye, firefighter Steve Newman said, but there were no other injuries.

Rescuers escorted four palace staff who had been working on the roof to safety, Newman said.

The palace said in a statement that a fire alarm system had detected the flames in an apartment above the East Gallery, a large corridor connecting the palace ballroom to the state rooms.

Spokeswoman Penny Russell-Smith told the BBC it was unclear how the fire started, but police said it was not set intentionally.

The state rooms are at the heart of the palace and are used regularly by the queen and other royals for entertaining.

The damage caused by Sunday’s fire did not appear to be anywhere near as bad as that done by two earlier palace blazes, at Windsor Castle in 1992 and at Hampton Court Palace in 1986.

Buckingham Palace said two areas of ceiling had been damaged and water from burst pipes soaked some carpets.

Firefighter Brian Robinson said there was also some minor smoke damage, but added that no royal treasures or artifacts had been harmed.

Sir Hugh Roberts, director of the royal collection, was inspecting artworks and other valuable items for signs of damage, and the palace said two paintings were removed as a precaution.

Eighteen royals and 12,000 guests attended a spirited classical music concert on the palace grounds Saturday night and excitement was already building for Monday’s show.

May was to open the concert by playing “God Save the Queen” from the palace roof and supporters around the world are expected to light a chain of 2,000 beacons, culminating in a display of fireworks launched from atop the palace.

The queen and her husband Prince Philip planned to attend a parade in Windsor earlier Monday, and hundreds of street parties were expected to begin around the country.