BELGRADE, MAY 13 -- More than 40,000 supporters of Serbian opposition parties took to the streets here today to denounce the hard-line Communist government of Serbian leader Slobodan Milosevic and demand free multi-party elections.

Shouting "Down with communism!" and "Slobo, traitor!" the crowd packed Republic Square in central Belgrade -- capital of both the Yugoslav federation and Serbia, the largest of the country's six constituent republics. Traffic in the area came to a standstill as demonstrators spilled into streets around the square, burning photos of Milosevic and waving anti-government placards and Yugoslav flags with the Communist star torn from the center.

The rally, organized by five newly formed opposition parties, was the largest anti-government demonstration yet seen in Serbia, whose leadership has adamantly resisted the drive toward political liberalization and separatism elsewhere in Yugoslavia that has threatened to bring about disintegration of the hybrid Slavic country.

Recent multi-party elections in the western Yugoslav republics of Slovenia and Croatia have brought non-Communist governments to power on platforms advocating greater local autonomy from the central government in Belgrade, or even complete independence if those demands are not met.

Authorities allowed today's demonstration to take place without interference, but denounced it as anti-Serbian. Until a few weeks ago, such an open display of contempt for the ruling Communists at the very heart of their power would have been unthinkable. The only sizable demonstrations in Belgrade over the last three years have been huge rallies orchestrated by the Serbian Communist Party to celebrate Serbian nationalism and the career of Milosevic.

Yugoslavia was created as a Communist-ruled federation in 1945 by the charismatic marshal Tito, a Croatian who used his stature as a Slavic folk hero to keep nationalist tensions between the country's numerous ethnic and cultural groups in check until his death in 1980. Since then, these hostilities have reemerged as leaders of the republics have fought for a share of power in the vacuum left by Tito's death.

At today's protest gathering, leaders of the opposition parties delivered a petition to the Serbian legislature demanding free elections in the republic before the end of the year. The Communist leadership has repeatedly rejected such demands, saying that the Serbian constitution must first be changed before elections can be held.

Milosevic and the Serbian Communists have been able to keep a tight grip on power in part because they were the first among the leaders of Yugoslavia's republics to exploit the rising nationalist fervor.

Milosevic and his supporters mobilized Serbian resentment against alleged abuse of their kinsmen in the Serbian province of Kosovo, where ethnic Albanians outnumber Serbs by 10 to 1. Bloody rioting and clashes plagued the province for months after Serbia imposed tight control over Kosovan police, courts and schools.

But now the very Serbian nationalism that was the foundation of Milosevic's power may prove his undoing, according to observers in Belgrade. As long as he was the supreme Serbian patriot, most of Serbia's 10 million people seemed tolerant of his communism, which many here feel has brought Serbia to economic ruin.

Now, however, the non-Communist opposition seems to have stolen some of his thunder. At today's demonstration, there were shouts of "We are going to Kosovo," and one speaker drew loud cheers when he declared: "Serbia's size gives her the right to talk to the other peoples of the Balkans from a position of force."

"By conjuring up nationalist hysteria," said prominent Belgrade lawyer Srdja Popovic, "Milosevic has let the genie out of the bottle. However hard he tries, it's too late to put it back in."