The planned mass rally is organised by the opposition nationalist Serbian Progress Party (SNS) which is hoping to cash in on growing public discontent as Serbia struggles with rising prices, inflation and high unemployment.
While the opposition is pushing for early elections, the government insists that elections should only be held after a European Union decision on Serbia's membership application expected by the end of the year.
They argue that an election campaign in autumn could slow down its EU accession.
"We should not jeopardize our candidacy by an electoral process," Serbian President Boris Tadic warned last week.
Media here are reporting that the government has proposed early elections in January but the SNS, which broke off from the ultra-nationalist Serbian Radical Party (SRS), has picked the date of 18 December.
SNS leader Aleksander Vucic told Vecernje Novosti daily that he would make the authorities an offer they cannot refuse if they do not set a date for early elections on the day of the rally.
"Our people will not just leave If (Tadic) fails to call elections we will make him an offer he cannot refuse," he said.
He would not give any details about the offer, saying it is a surprise. He added that he fully expects the government to agree to their demands when they see the size of the protest.
The SNS predicts there will be more than 100,000 people. A similar rally in February attracted over 70,000 protesters.
With elections officially only due by May next year Vucic, whose party now holds only 21 of the 250 seats in parliament, is riding high on a wave of opinion polls which put the coalition around his party in the lead.
Political analysts question what the rally can bring the opposition, adding that its also risky for them.
"The rally is a form of pressure on the authorities to call early elections. However, the opposition clearly knows that they cannot get the election date when they want but at a moment when the ruling coalition estimates it will benefit from it," analyst Marko Blagojevic of the CESID think tank which monitors elections in Serbia told AFP.
He added that the protest also served as a show of strength from the SNS which they could use to attract potential coalition partners.
"The chances of the SNS to get early elections by way of protest are not insignificant but when you organise a rally at a point when dissatisfaction is reaching a boiling point you never know what will be the result," independent analyst Vladimir Goati warned.
In the last few months the Serbian government has faced mounting social unrest as doctors, teachers, policemen and other public sector workers called strikes and demonstrated for pay rises and better working conditions.