They have also called for round-table talks with President Slobodan Milosevic's Socialist Party.

Representatives of the two main opposition wings - the Alliance for Change and the Serbian Renewal Movement (SPO) - signed a deal on the conditions for holding fair elections.

But a representative of the smaller opposition Vojvodina coalition refused to do so, saying the demands were too weak.

The joint document included conditions on a proportional voting system, revision of election lists, the presence of domestic and foreign observers, and free media.

Vuk Obradovic, left, protects himself from police at an anti-Milosevic rally on Wednesday

"Electoral conditions have been agreed," said Vuk Obradovic, leader of the Social Democracy party.

"We will demand a round table meeting with authorities as soon as possible and the organisation of early general elections in Serbia as soon as possible."

Yugoslav elections are due next year, but presidential and parliamentary elections for its dominant republic Serbia are not due until 2001 and 2002 respectively.

The ruling parties have said that early elections are not a priority, but that they would be ready to hold a ballot if the opposition officially requested it.

Long-time division

The main opposition parties have previously called for early elections, but up to now they have disagreed on whether a fair vote is possible while Mr Milosevic remains in power.

The main axis of rivalry is between Alliance leader Zoran Djindjic and SPO leader Vuk Draskovic.

A man waves a WW1-era Serbian flag at an anti-Milosevic rally in September

Mr Draskovic wanted to work within the existing system, which would involve political cohabitation with Mr Milosevic. But Mr Djindjic demanded fundamental change in Serbia, starting at the top.

On September 21, the Alliance launched a nationwide campaign to force Mr Milosevic to step down. But the rallies have so far failed to attract significant numbers of protesters.

The SPO has not taken part, describing the daily protests as a waste of time because Mr Milosevic can only be defeated at the ballot box.

In another sign of opposition parties closing ranks, their officials earlier this week agreed a joint statement to the 15-nation European Union suggesting that elections be held by early 2000.