BELGRADE, Aug. 19—In a massive outpouring of opposition sentiment, more than 150,000 Serbs took to the streets of this capital tonight to demand the resignation of President Slobodan Milosevic and call for democratic change in Yugoslavia. 
Ignoring last-minute political maneuvers by the government and veiled warnings of violence aimed at limiting attendance, the crowd clogged a vast area in front of the Yugoslav federal parliament building, waved opposition party flags and chanted slogans against Milosevic. 
The rally, the largest anti-Milosevic demonstration in Belgrade since a series of protests against alleged election fraud in late 1996 and early 1997, went ahead peacefully despite some tense moments shortly after it started when a tear-gas grenade exploded in the crowd and briefly scattered protesters. They quickly regrouped, however, and there were no further incidents or reports of injuries. 
Under a half moon on a clear, warm night, speaker after speaker called for the resignation of Milosevic, who has held power here for the past decade, first as president of Serbia and since 1997 as president of the Yugoslav federation. Serbia is the dominant republic in the federation, which also includes Montenegro. 
Among the speakers was Vuk Draskovic, the mercurial leader of the largest opposition party, who had had a change of heart after earlier canceling his speech in a dispute over the appearance of a rival politician. 
The demonstration was widely anticipated here as a barometer of the public's attitude toward Milosevic in the aftermath of his disastrous policy in Serbia's southern province of Kosovo, which led to a 78-day NATO bombing campaign, the withdrawal of Serbian forces and the deployment of a NATO-led peacekeeping force. 
Although the crowd exceeded organizers' expectations, it was unclear where the protest movement will go from here. The opposition remains fragmented, and Milosevic has survived previous attempts to force him out. 
In their speeches tonight, opposition leaders vowed to keep pressuring Milosevic to step down. 
"The people are getting ahead of their leaders," said accountant Radisa Lejic, 45, referring to opposition politicians whose squabbling had threatened to reduce the turnout. "This is the last minute for something to be done, and if we don't do it, then who else can?" 
The government-controlled television network, however, called the rally "a failure in every sense." It said the event was ordered by the opposition's "NATO bosses" and that Draskovic had shown up "to fulfill a promise" to Robert S. Gelbard, the State Department official who oversees U.S. Balkans policy. 
"Slobo odlazi," literally "Slobo go away," the crowd frequently chanted, holding up a three-finger gesture of Serbian patriotism based on the Holy Trinity of the Serbian Orthodox Church. 
"Red bandits!" yelled angry demonstrators after the tear gas exploded, a reference to the communist origins of Milosevic's ruling Socialist Party. 
"We want to go back to Europe, where we were before communism and neo-communism," said Bishop Artemije, an influential Serbian Orthodox cleric representing Kosovo, in a message read for him from the stage. 
In another message to the crowd, Aleksandar Karadjordjevic, the London-based crown prince of Yugoslavia, urged opposition parties to stay out of any coalition with Milosevic, adding that "the salvation of Serbia depends on this moment." Royalist supporters lit Roman candles and sang the Serbian royal anthem. 
"Milosevic, look at this square," said Zoran Djindjic, a leader of the Alliance for Change, a coalition of opposition parties. "This is where your citizens are. These are citizens who don't want you. Go away before we remove you." 
If Milosevic does not resign in 15 days, he said, demonstrations will be held across Serbia until he does. "Now it's us or them," Djindjic said. "It's Milosevic or Serbia. There is no third way." 
Draskovic, who heads the Serbian Renewal Movement, sounded a more conciliatory note. He criticized other opposition leaders' demands for a transitional government, saying that "we cannot make a government in the streets," and he called for new elections by the end of November whether Milosevic presides over them or not. He thus appeared to be accepting offers by Milosevic's coalition to hold elections by then in what is seen as an attempt by Milosevic to ease the pressures for his resignation. 
Many supporters of other parties booed the bearded firebrand as he delivered his oration, reflecting the ongoing divisions within the anti-Milosevic camp. 
Draskovic and other speakers criticized the presence of NATO-led troops in Kosovo, where they said that Serbs who are supposed to be under the force's protection are being kidnapped and killed by ethnic Albanians. Most of Kosovo's prewar Serbian population has fled the province over the past two months. 
In an apparent attempt to limit the rally's turnout, authorities canceled a number of buses and trains into Belgrade from Serbian cities and warned that "unknown" perpetrators could try to plant explosives at large public gatherings. 
On the eve of the rally, Milosevic's Socialist Party also floated the idea of holding early elections, possibly by November. Spokesmen for the party and its coalition partners said they were "ready" for new elections any time and challenged opposition parties to name a date. 
The move appeared to reflect a calculation that the opposition parties may be too deeply divided to win -- and that the situation can only get worse for Milosevic the longer he waits to take action. The government especially dreads the onset of winter, when the country is expected to face a heating crisis because of a shortage of electricity. 
Political analysts also see the offer of early elections as a ploy to buy time so Milosevic can further undermine the opposition and defuse pressure on him to resign. Opposition parties are split on conditions for new elections, with some insisting that they should be held only after Milosevic steps down and once changes are made to election and media laws that favor his government. 
According to a recent poll by Belgrade's Partner agency, opposition groups have been gaining in popularity, but are still far from receiving majority support. If an election were held today, the poll of 1,000 citizens in 32 municipalities said, Draskovic's Serbian Renewal Movement would get 18.1 percent of the vote, and the rival Alliance for Change would receive 14.8 percent. The three parties in Milosevic's ruling coalition would get a total of 24 percent, the poll indicated. Nearly 30 percent of those polled were either undecided or would boycott the election.