The international community has called for urgent changes in Serbian law after Sunday's presidential election failed to muster enough votes for the poll to be valid.

Only 46% of voters took part in the run-off between current Yugoslav President Vojislav Kostunica and his rival, Miroljub Labus.

Mr Kostunica won a heavy majority of the votes cast - but without the minimum turnout of 50%, the result was void.

It means the entire two-round poll will have to be re-run, possibly extending the uncertainty for months and deepening the country's political crisis.

Votes trickled in but failed to pass the 50% threshold

The 50% rule - introduced by ousted leader Slobodan Milosevic - was attacked by the Organisation for Security and Co-operation in Europe (OSCE), which monitored the elections.

In a strongly-worded statement, the OSCE said the law should be repealed urgently.

The OSCE said the law allowed minor parties to call on their supporters to boycott elections, which undermined voter confidence.

One senior Western diplomat in Belgrade told the BBC there would be "heavy pressure" in the coming days on the Serbian Government to change the law.

Serbia's voters - who took to the streets in their thousands to oust Mr Milosevic - are now thought to have grown disillusioned with the politicians who replaced him.

The constant squabbling and arguments among parties must affect voter willingness

Zoran Nikolic, Belgrade voter

The BBC's Matthew Price in Belgrade says that two years ago Serbians voted for a reform-minded government, but feel little real progress has since been made.

The election has also highlighted the bitter power-struggles between different Serbian factions.

Mr Kostunica is strongly opposed by current Serbian Prime Minister Zoran Djindjic, who backed Mr Labus in the failed poll.

Mr Kostunica accused Mr Djindjic of helping keep turnout low by failing to update electoral registers, keeping tens of thousands of people from voting.

I will continue to fight for a legal change in Serbia, so things like this, the shame before the world, will never be repeated again

Vojislav Kostunica

Mr Kostunica also faces bitter opposition from hardline nationalist leader Vojislav Seselj, who was eliminated in the first round but performed very strongly.

Mr Seselj, who is backed by Mr Milosevic, will hope to profit from the election rerun by improving the 22% share which he took first time around.

Voters confirmed that they were weary of the bickering between the leaders who had worked together to oust Mr Milosevic, but have since split over the pace of reforms.

"The constant squabbling and arguments among parties must affect voter willingness," said Zoran Nikolic, 47,as he cast his vote in Belgrade.

The poll's failure creates international embarrassment for Serbia as it tries to build on its improved relations with the European Union and Nato.

It will jeopardise our image if we don't have a president of the state. That's something no country is proud of

Miroljub Labus

Mr Kostunica called the failure a "shame before the world" which should never be repeated.

He also vowed to fight to remove what he termed the "anti-electoral" 50% turnout rule.

Mr Labus, currently a Yugoslav deputy prime minister, agreed that Serbia's international standing would suffer.

"It will jeopardise our image if we don't have a president of the state. That's something no country is proud of," he said.

Mr Kostunica blamed the election failure on boycott calls by some politicians. He blasted the boycott as "extremely anti-European".

Reforms under threat

Western diplomats say they are concerned that Serbia's politicians could spend months occupied with a fresh election, instead of focusing on economic and political reforms.

The post of Serbian president is not a powerful one on paper - the prime minister wields the real power in the land.

Kostunica got most votes, but didn't win

But at stake is major influence over the issue of co-operation with the war crimes tribunal in The Hague.

The new president will replace Milan Milutinovic, who has been indicted by the United Nations war crimes tribunal, where Mr Milosevic is on trial.

Mr Milutinovic was deemed immune from prosecution until his term expires at the end of this year.

The poll's failure was lamented by parts of the Serbian press.

"We voted in vain," said the front-page headline in Glas Javnosti newspaper.