Several hundred Serbs who had fled Kosovo took part in Monday's demonstration in the central square of the capital to call for protection by the United Nations from reprisals by Kosovo Albanian rebels.

Organisers Svetozar Sisic and Slovodan Karalajic were arrested at the demonstration, immediately put on trial and jailed for 30 days, according to sources from the Democratic Party of Serbia.

Thousands of Serbs have fled Kosovo in terror

Mr Sisic had called on all 50,000 Serb refugees from Kosovo to gather in front of the Assembly of the Federal Republic of Yugoslavia "to express their protest at what has happened to us".

Most Serb refugees who have tried to enter the capital have not been allowed to, and are camped outside its boundaries.

The authorities have prevented other demonstrations against the government under the state of war declared throughout the country.

Call for change

On Monday, the opposition Alliance for Change umbrella group said it would start organising demonstrations across the Yugoslav republic calling for early elections, and democratic change.

"This is the last minute to reverse the present political course in Serbia, and to demand the responsibility of those who have had unlimited power in the decision-making process," Alliance official Milan Protic said.

Returning Albanians watch as Nato soldier checks KLA woman for weapons

Officials in Belgrade have been appealing for Kosovo Serbs not to leave the province, in spite of fears for their safety amid revenge attacks and looting by ethnic Albanians.

Nato, which entered Kosovo pledging to act evenhandedly for the protection of both ethnic groups in the province, said about 2,000 Serbs had returned to Kosovo on Sunday and Monday.

Milosevic under pressure

The demonstrations in Belgrade come as an embarrassment for Yugoslav President Slobodan Milosevic, highlighting the flight of about Serb civilians from the province, which he insisted remain under Belgrade's sovereignty.

BBC South-East Europe Analyst Gabriel Partos says the opposition remains far from united, and Mr Milosevic has always been skilful at exploiting divisions among his opponents.

This time though, Mr Milosevic is under pressure because of his defeat at the hands of Nato, and the arrival of Serb refugees from Kosovo in Belgrade.

Correspondents in Belgrade say the police at Monday's demonstration appeared tense and ashamed at having to oppose the crowd.