SOFIA, BULGARIA -- Thousands of protesters demanding immediate removal of Communist symbols from public display stormed the headquarters of the governing Socialist Party yesterday, ransacked the building and set it afire.

Government officials initially ordered police not to intervene, but late last night security forces sealed off roads into the capital and declared a national alert.

In a special radio brodcast, Socialist President Zhelyu Zhelev described the rampage as "a senseless act," warned that the country was in danger of "drifting . . . into a military dictatorship" and said measures would be taken to prevent anarchy and civil war.

The eight-story granite building, the largest in Sofia, was for years headquarters of the former Communist Party, which changed its name to the Socialist Party after government reformists ousted hard-line Communist leader Todor Zhivkov last fall.

The fire burned for an hour before firefighters could even make their way through the crowd to turn hoses on the flames and then continued late into the night.

"This is our storming of the Bastille," said a political opposition leader among about 15,000 chanting protesters who watched the blaze consume the ground floor of the building.

The incident, prompted by an official campaign sanctioned last week to remove symbols of Stalinist rule from the city, was the most serious in Bulgaria since Zhivkov's ouster.

Countless flags, stars and hammers and sickles had been taken down since then, but the crowd was apparently provoked because officials had not removed a 90-foot-high illuminated red star that dominates the skyline from atop the party building.