The protestors turned out to hear maverick politician Vuk Draskovic of the Serbian Renewal Movement say the country's leaders "must become the past so that our country can have a future".

The crowd, the largest of its kind since the end of the conflict in Kosovo, responded with chants of "Slobo go! Slobo go!".

The demonstrations come as Yugoslav President Slobodan Milosevic blamed Nato and the United Nations for the killing of 14 Serbs in the village of Gracko in Kosovo on Friday night.

Relatives of the murdered Serb farmers had to identify the bodies

Serb villagers said the Kosovo Liberation Army had carried out the killings.

The Yugoslav war crimes tribunal at the International Court of Justice in The Hague has ordered an investigation into the massacre. The court's chief prosecutor, Louise Arbour, said the strongest deterrent message must be sent to those who wanted to perpetuate the cycle of violence in the province.

Nato and UN officials condemned the killings, the worst attack on Serbs since the arrival of the peacekeepers in June, as inhumane and appealed for calm.

Ambitious demonstration

In Nis, Mr Draskovic repeated proposals that Serbia be run by a transitional government that could guide the country to elections.

Jacky Rowland: Vuk Draskovic's most ambitious meeting to date

Responding to recent comments by military leaders that they would prevent any attempt to overthrow the current government, he said generals had nothing to do with internal political discussions.

The demonstration marks the second Saturday running that Mr Draskovic has called opponents of the government out onto the streets, but this meeting was more ambitious than the last.

Rival group

Supporters of the Alliance For Change braved the rain in Sombor

In the town of Sombor, near Serbia's northern border with Hungary, rival opposition group Alliance For Change attracted a crowd of about 4,000 to hear proposals for a former central bank governor to take over as head of a new interim government.

The BBC's Belgrade correspondent, Jacky Rowland, says it is clear the opposition is fragmented and has not attained a critical mass.

Different interest groups are pressing their particular grievances in an uncoordinated way.

Our correspondent says unless the political opposition to President Milosevic organises itself into a united force, these demonstrations will continue to be ineffectual and ultimately meaningless.