CAIRO, Egypt (AP) _ A bomb exploded today under an empty tourist bus outside the Egyptian Museum on Tahrir Square, one of Cairo's largest and busiest public spaces. Police said there were no casualties.

The blast outside Egypt's biggest museum, home to thousands of pharaonic items including the famed King Tut gold treasures and royal mummies, broke the bus windows and blew out a luggage compartment. Windows in five empty buses parked nearby also were shattered.

There was no immediate claim of responsibility. But a militant Islamic group has been attacking tourism targets in an effort drive away foreign visitors and destablize Egypt's secular government by depriving it of an important source of income.

The extremists, led by radical cleric Omar Abdel-Rahman who is in self- exile in the United States, want to install an Iranian-style Islamic theocracy. Two men who worship at the mosque where Abdel-Rahman preaches have been charged in the Feb. 26 bombing of the World Trade Center in New York.

The bus bombing came a day after police arrested 150 Muslim radicals in a raid on a small island in the Nile River near Assiut, a center of extremist violence about 200 miles south of Cairo. One policeman was reported killed in the raid, the latest in a nationwide crackdown on the militants.

Witnesses said the tourists who had been on the stricken bus were inside the museum when the bomb exploded. Most tourists leaving the museum stopped to look at the damaged bus and take pictures.

''We heard a bang. First we thought something fell, but then 15 minutes later we came out and saw the crowds and police and realized it was a bomb,'' said John Traynor, a tourist from Scotland.

It was the second blast on the square this year. On Feb. 26, a bomb in a coffee shop killed three people and wounded more than 15. That bombing was blamed on Muslim extremists.