Ukraine's opposition parties are holding a rally in Kiev in protest at what they describe as "fraudulent" parliamentary elections last month.
The protesters say the much delayed vote-count is being rigged in favour of pro-government candidates.
The authorities deny the claim, warning that police are ready to disperse the "illegitimate" rally in the capital.
International observers say the election was marred by "abuse of power and the excessive role of money".
The monitoring mission from the Organisation for Security and Co-operation in Europe (OSCE) say state resources were used to give the party of power an unfair advantage during the campaign, and note that President Viktor Yanukovych's chief rival, former Prime Minister Yulia Tymoshenko, is in prison and was barred from running.
However, the mission adds that the actual process of voting on 28 October went smoothly.
With nearly 100% of the votes counted, Mr Yanukovych's Party of the Regions (PR) looks on track to become the largest party, poised to have a majority in the new parliament with the support of its Communist allies and independent candidates.
Warning to president
Thousands of opposition supporters are staging the protest rally in front of the election commission headquarters in central Kiev, amid a heavy presence of elite anti-riot police units.
The protesters are chanting "Down with Yanukovych! Down with the gangster!" and "Shame!"
Arseniy Yatsenyuk, one of the leaders of the united opposition, said that election officials were "stealing votes and rewriting results".
He warned that President Yanukovych "bears personal responsibility for the fact that the number of votes is being falsified".
The demonstrators say the vote-count is being rigged in a number of disputed regions, where the opposition appeared to be winning.
In one district in southern Ukraine, police last week used tear gas as tensions boiled over between representatives of the rival camps.
In protest, the opposition have threatened to make the new parliament void, by handing over their mandates.
The authorities deny the opposition claims of vote-rigging, saying they are trying to intimidate election officials.
The government also warns that police will act to disperse Monday's rally, which has not been authorised.
Under Ukraine's election law, half of 450 MPs are elected on a party list basis and half in single-seat constituencies.
With 99.9% of the party list vote counted, the PR has 30% to nearly 26% for the united opposition, which includes Mrs Tymoshenko's Fatherland party.
Another two opposition parties, including boxing champion Vitali Klitschko's Udar, are together polling about 25%.
With 99.05% of the single-seat vote counted, the PR has taken a commanding lead, securing 115 seats.
The party is widely expected to be supported by most of 44 independents, who the opposition accuses of being de facto pro-government candidates.
The opposition parties have so far secured around 60 seats.