We will continue with the rallies over the weekend and start a general strike on Monday to grind Serbia to a standstill

Opposition spokesman Zoran Djindjic

But the turnout was much lower than at a rally on Wednesday night, which drew a crowd of about 200,000.

The authorities say Mr Kostunica was just short of a majority and so there has to be a second round run-off vote.

But Mr Kostunica has called for an internationally-monitored recount of the vote.

Country-wide protests

Elsewhere in Serbia, roads were blocked, students walked out of schools and businesses closed as the opposition campaign to force President Milosevic to resign got under way.

About 20,000 people attended a protest rally in the southern city of Nis, where students marched through the streets chanting slogans like: "Milosevic, kill yourself and save Serbia".

Vojislav Kostunica has ruled out taking part in a second round

Large crowds also gathered in the key northern city of Novi Sad.

In Cacak, Krusevac and Valjevo, students were joined in the streets by workers. In various parts of Serbia strikes were staged even at state industries previously loyal to President Milosevic.

Western leaders have added their voices to the opposition's demand for Mr Milosevic to step down.

The US State Department spokesman, Richard Boucher, said the opposition had "presented a persuasive case that they won in the first round". There was no need for any recount, he added.

Kosovo election 'lie'

Meanwhile, the UN administrator for Kosovo, Bernard Kouchner, said the figure of 140,000 votes for President Milosevic in Kosovo was a "lie".

The UN mission had witnessed no more than 45,000 voters going to the polls in the province, he said.

The opposition campaign comes amid growing signs of dissent within Mr Milosevic's ruling establishment, with indications from the army that it would not intervene against the people.

The church has thrown its weight behind the opposition

However, the opposition suffered a setback on Friday when the army refused to meet their representatives to discuss the election results.

Russia has announced it is sending its special Balkans envoy, Vladimir Chizhov, to talk to both sides on Monday.

There have been rumours in Belgrade that President Milosevic may be planning an escape route to Russia.

But the BBC's Belgrade correspondent, Jacky Rowland, says such a scenario still appears some way off.

Expulsion

In another development, the authorities have ordered the BBC correspondent to leave Yugoslavia within 48 hours. They accused her of biased, pro-opposition reporting.

The BBC is protesting against the order to the Yugoslav authorities.

A BBC spokesman said the corporation was still hopeful that the decision would be reversed.

Mr Milosevic's Socialist Party of Serbia (SPS) attempted to counter talk of defeat for its leader by announcing that the SPS and its two allies had won a majority in voting for a new parliament, which was held alongside the presidential polls.

Official results

Vojislav Kostunica: 48.96% or 2,474,392 votes Slobodan Milosevic: 38.62% or 1,951,761 votes 7,200,000 registered voters 69.7% of voters cast ballots

Source: Federal Election Commission

SPS leaders said they would form the next Yugoslav federal government.

However, even Mr Milosevic's long-time ally, the ultra-nationalist leader Vojislav Seselj, has recognised Mr Kostunica as the first-round victor.

"For us, the elections are over. According to our data, Kostunica has won in the first round and thus we will not take part in the run-off," Mr Seselj said.