Opposition groups today demanded that Yugoslav President Slobodan Milosevic leave office within a month and vowed to shut down the country with continual demonstrations, road blockades and a general strike if he does not.

Leaders of the Alliance for Change, an opposition coalition that helped organize a massive street rally against Milosevic Thursday night in Belgrade, said they would hold their next major demonstration in the capital on Sept. 21.

"From that day on, there will be no going home," said Vladan Batic, the alliance's coordinator. At a news conference with other coalition leaders, he said the group would call for people to stage continual protests all over Serbia, Yugoslavia's dominant republic, and carry out various acts of civil disobedience if Milosevic remains in office.

The Milosevic government did not immediately respond to the opposition demands or to Thursday night's protest, but government-controlled news media sought today to play down the size of the demonstration, paint its organizers as pawns of the United States and NATO and emphasize splits in opposition ranks. The newspaper Politika, considered a Milosevic mouthpiece, called the gathering a "NATO rally," said it had been a "failure" and published a photograph of protesters fleeing a tear gas grenade that exploded in the crowd shortly after the event started.

While the rally revealed an opposition still deeply divided, with bickering leaders pursuing divergent paths to power, the size and enthusiasm of the crowd also illustrated Serbs' strong desire for change. Members of the throng, estimated at more than 150,000, made it clear that they want Milosevic to quit regardless of the disagreements and personal feuds among opposition politicians.

"The people who came to the rally last night showed they didn't come because of the leaders, but . . . mostly because of themselves," said Mladan Dinkic, whose Group of 17, an organization of independent economists, conceived the demonstration. "They are forcing both the opposition and the regime to think about their positions." The Group of 17 wants Milosevic to step down in favor of a transition government that would restore "stability" in Serbia and pave the way for new elections.

Vuk Draskovic, who heads the opposition Serbian Renewal Movement, insisted on pursuing new elections even without a transition government and dismissed the booing and jeering that greeted his speech at the rally, which he attended despite having canceled his appearance earlier.

"If they don't bring [Milosevic] down in a month, we are going to bring him down in elections," Draskovic said of the deadline set by the Alliance for Change. He added, "Even if nine million Serbs boo me . . . I will not allow civil war. I won't, I won't."

At the Alliance for Change news conference, Democratic Party leader Zoran Djindjic said that if Milosevic did not agree to a transition government in 15 days, the coalition would go ahead with its plans for the Sept. 21 rally and strike. And if Milosevic did not step down by then, he said, the protests and civil disobedience would continue daily.

Ognjen Pribicevic, a top adviser to Draskovic, said the 15-day deadline was "obviously not serious" and that "we think only our strategy can be successful." He summarized that as reaching out to disaffected Milosevic supporters and working for early elections.

"If Djindjic can overthrow [Milosevic] in 15 days, we'd be very grateful to him," Pribicevic said. But he said he sees no chance that Milosevic would simply give way to a transition government. He added that the Serbian Renewal Movement does not plan to participate in the Sept. 21 demonstration.

CAPTION: Students commemorate Slobodan Milosevic's 58th birthday by burning nine-slice cardboard cake representing areas lost in four wars he has fought.