The Ukrainian cabinet has cancelled a planned meeting after the opposition barred Prime Minister Viktor Yanukovych from entering the government compound.
Yushchenko supporters blockaded the government compound
Earlier, the opposition leader Viktor Yushchenko urged his supporters to prevent the meeting of what he called the "illegal government".
Mr Yanukovych has officially contested the results of the re-run presidential election that took place last Sunday.
Preliminary results show Mr Yushchenko won more than half of the votes.
But he will not be proclaimed the president until his rival exhausts all legal possibilities to contest the results.
Mr Yanukovych filed his complaint to the election commission, which now has two days to examine it. If it turns it down, Mr Yanukovych can appeal to the Supreme Court, which he has vowed to do.
The meeting of the Ukrainian cabinet, still headed by Mr Yanukovych, was postponed after pressure from Yushchenko supporters, who had responded to their leader's call to blockade government buildings.
"In what country is it possible for a government that has been dismissed to say that it doesn't want to go? No meeting of an illegitimate government can take place," told thousands of supporters in Kiev.
Mr Yanukovych has refused to accept the result, alleging thousands of irregularities.
The election was re-run after Ukraine's courts ruled the first poll was illegitimate. That followed protests by hundreds of thousands of Ukrainians, and Western governments, that the ballot had been rigged in Mr Yanukovych's favour.
On Tuesday Mr Yushchenko, a pro-Western liberal reformer, said it would take time to rebuild relations with Russia, which openly backed his rival.
"I really disliked the fact that Russia campaigned in favour of one candidate," Mr Yushchenko told the Russian daily Izvestia.
"This was a serious wound for millions of Ukrainians," he said.
Viktor Yushchenko's supporters have thronged the streets of Kiev
International observers said the re-run was much fairer than the earlier vote.
But the Russian foreign ministry challenged the objectivity of European election monitors from the Organization for Security and Co-operation in Europe (OSCE).
"Violations characteristic of the first and second round of voting were repeated," said ministry spokesman Alexander Yakovenko.
Despite the tensions, Russian Defence Minister Sergei Ivanov said Mr Yushchenko's victory would not have "any significant impact on our co-operation... in the defence and security field".
Mr Yanukovych's decision to appeal against the result could revive the legal wrangling over the election.
However, the crowds of Yushchenko supporters on the streets of Kiev are not allowing the continuing doubts to dampen their victory celebrations, the BBC's Steve Rosenberg says, but are rallying with their trademark orange scarves and banners.