After sitting on the sidelines for weeks, Serbia's most powerful opposition leader, the charismatic and bombastic Vuk Draskovic, called on his supporters today to join him in "massive rallies all over Serbia" as part of the growing movement to force Yugoslav President Slobodan Milosevic from office.

The public pressure on Milosevic -- which has been building since he agreed last month to withdraw Serb-led government forces from Kosovo and let NATO peacekeepers in -- is entering a new, potentially explosive stage. Opposition figures are warning darkly of a possible bloody showdown or civil war, just as Milosevic's allies are heightening their rhetoric, calling government opponents and some media figures spies, traitors and lackeys of the CIA.

Draskovic, the self-styled "czar of the streets," who drew tens of thousands of people into central Belgrade 2 1/2 years ago to demonstrate against the Milosevic government, had avoided overtly supporting protest rallies led by other opposition figures in several Serbian towns and cities over the past two weeks.

He said today that the wait-and-see period is over and that he is prepared to confront the Milosevic government with huge street protests beginning Saturday in the central city of Kragujevac.

Draskovic's participation adds a new dimension to the anti-Milosevic drive. His Serbian Renewal Movement, the second-largest party in Serbia after Milosevic's Socialist Party, controls the city government of Belgrade and, perhaps most importantly, operates an influential television station called Studio B.

In this tightly controlled society, a television station can be a crucial tool of repression or of revolution. Thus far, Studio B -- like the pro-Milosevic state-run television stations -- has provided little or no coverage of demonstrations organized by other opposition leaders, but it is expected to cover Draskovic's first rally in great and sympathetic detail. Since no other opposition figure has control of a television station, the broadcast propaganda war will be fought by only two combatants, Draskovic and Milosevic.

In the weeks before, during and immediately after the NATO bombardment of Yugoslavia, Draskovic offered mixed signals of his political intentions. He had served for a time as a deputy premier in the Milosevic government and was critical of the NATO air campaign, but he was forced to resign after declaring publicly that Milosevic had lied to the nation by claiming that government forces were winning the war in Kosovo, a province of Serbia, Yugoslavia's dominant republic.

In an interview today, Draskovic vowed not to repeat his mistakes of years past, when he spoke in romantic terms of a return of the Serbian monarchy -- not to rule, but as a symbol of the state. This time, he said, he is going after the votes and voices not only of his core supporters, but also of those Serbs who have supported Milosevic.

Unlike other opposition leaders, Draskovic asserted that it is not productive to threaten Milosevic and his inner circle with arrest for alleged war crimes against ethnic Albanian civilians in Kosovo. He said that talking about revenge and arrests was a way to lose potential supporters who have sided with Milosevic and supported his nationalist policies for the past 10 years.

By calling for anti-government demonstrations, however, Draskovic is moving toward open conflict with the Yugoslav leader -- a maneuver that could backfire if Milosevic and his government were to respond by attempting to strip his party of power in Belgrade or to seize control of its television station.

Moreover, any effort to topple Milosevic could be handicapped by continued squabbling among opposition leaders. A ready illustration of this is the fact that the leader of the opposition Democratic Party, Zoran Djindjic, has also called for an anti-government demonstration in Kragujevac -- on the day before the Draskovic rally.

Even as they seek to play down their differences, Draskovic and Djindjic took shots at each other in separate interviews today. Djindjic said, nonetheless, that his party, along with other opposition groups, plans to stage up to 20 protest rallies around Serbia in coming weeks, culminating in a mass rally in Belgrade in August. Draskovic's plans also include large-scale street demonstrations in Belgrade.

Djindjic acknowledged that the public mistrusts all politicians and parties and simply wants someone to lead them toward change. "What we have had this last 10 years," Djindjic said, referring to a decade of Milosevic's rule in Serbia and Yugoslavia, "is a kind of sick political life. What the people want now most is something fresh, something that says, okay, here is the first step, and we will figure out the rest as we go."

In Belgrade today -- after they were blocked by police on Monday -- youthful members of the opposition group Alliance for Change began gathering signatures on a petition calling for Milosevic to step down. At a table set up on a main boulevard, thousands of people signed.

"This is my country, my city, and I think everyone should sign this paper," said Goran Djordjevic, an unemployed laborer. "I say what everyone should say, that things must change, and soon."

Support for the effort was not unanimous, however. One of the young signature collectors, Branimir Kuzmanovic, said that several men had ripped down their signs and flags and punched two of his colleagues. "But we did not want to incite them," he said. "There are all sorts of lunatics now, on the left and on the right." Kuzmanovic said he just wants "normal people" to sign the petition.

CAPTION: Vuk Draskovic issues a public challenge to Milosevic. (Photo ran in an earlier edition)

CAPTION: Belgrade residents sign a petition demanding the resignation of Yugoslav President Slobodan Milosevic. "I say what everyone should say, that things must change, and soon," said one of the thousands who came forward.