BELGRADE - Defying police intimidation and fierce government propaganda, about 150,000 demonstrators Thursday demanded Slobodan Milosevic's resignation in the biggest outburst of popular resentment against the autocratic Yugoslav president in years. 
The rally was a major test for the fragmented opposition in its efforts to unseat Mr. Milosevic, who has led his country into four disastrous wars that have left it in economic ruin. 
''The people have stood up and will not sit back till the changes are made,'' the opposition leader Zoran Djindjic told the cheering, flag-waving crowd. ''Milosevic must go for Serbia to be free.'' 
''This time we'll go all the way. It's either him or us,'' Mr. Djindjic said, giving the Yugoslav president 15 days to resign or ''the whole of Serbia will rise up.'' 
Mr. Djindjic was joined on the platform by Serbia's other main opposition leader, Vuk Draskovic, who heads the Serbian Renewal Movement. Mr. Draskovic's appearance was unexpected as he had initially said he would not attend the rally because he disagreed with the views and aims of some other participants. 
Mr. Draskovic, who served briefly in the government earlier this year before rejoining the opposition, called for elections under international supervision. 
Referring to Mr. Milosevic's pariah status internationally, Mr. Draskovic said, ''Serbia is in jail.'' 
''We are in jail,'' he said, because Serbia ''is led by those who are totally isolated by the world.'' 
''They must go to the political past so Serbia goes to the future,'' Mr. Draskovic said, as the crowd roared with approval. But he also denounced other opposition parties for making ''unrealistic'' plans for ''some transitional governments'' that no one will recognize. 
Such criticism, however, was not well-received. As Mr. Draskovic left the speaker's stand, he was booed and jeered by many in the crowd, some of whom threw old newspapers at him and his bodyguards. 
Riot police were deployed on side streets, but were not visible in front of the Parliament building. A senior figure in the Serbian Orthodox Church, Bishop Atanasije Rakita, opened the rally by reading out a letter from Yugoslavia's pretender to the throne, Crown Prince Alexander Karadjordjevic, who lives in exile in London. 
''You are here to demand radical changes,'' the crown prince said in the letter. ''The fall of Milosevic is only the first step. Don't miss this moment.'' 
Father Atanasije told the crowd: ''This is our only chance. If we miss it, God help us.'' At every mention of Mr. Milosevic's name, the crowd responded with ear-splitting boos and jeers. 
In an apparent attempt to keep people away, Belgrade police said Wednesday that they arrested a man with a ''highly explosive device'' and warned of possible bombing attacks at ''massive public gatherings.'' 
The police statement was prominently read several times during main state-run TV news, with an additional warning to parents to keep their children inside during the planned rally. 
On Thursday, Serbian television, under strict Milosevic control, also accused the United States of planning ''terrorist actions'' in Serbia, the main republic of Yugoslavia. 
As the rally began, a tear-gas canister was hurled near the speakers' stand, triggering a brief stampede. One man was carried from the scene, apparently injured. The rally continued despite the strong smell of tear gas. 
Several opposition parties organized the rally, the first since the 78-day bombing of Yugoslavia by the North Atlantic Treaty Organization that ended in June, calling for a transitional government of experts to replace Mr. Milosevic's government. 
The emotional crowd carried banners reading, ''Slobo, Please Just Go'' and ''You Sold Out Serbia.'' They held placards reading, ''Resignations'' and chanted, ''Slobo Go'' and ''Red Bandits.'' 
The authorities had launched a campaign against the rally, claiming it was intended as a celebration of President Bill Clinton's birthday Thursday and in support of NATO's ''occupation'' of Yugoslavia, meaning the international peacekeeping mission in Kosovo. 
Ivica Dacic, the spokesman for Mr. Milosevic's Socialist Party, said Thursday he expected ''nothing'' to come of the rally because the opposition had no popular support. 
''People think of those who will speak at the rally the same way as they think of those who have bombarded Yugoslavia,'' Mr. Dacic told reporters before the protest. 
In an apparent attempt to defuse tensions that have threatened Mr. Milosevic's rule, the Socialist Party on Wednesday offered to hold early elections. Regular elections are scheduled for 2001. But fearing fraud, the opposition is likely to reject the offer unless Mr. Milosevic agrees to international monitoring of the vote.