Ukraine's parliament has declared last Sunday's presidential poll invalid.
Thousands of opposition supporters are camped outside the building
Members overwhelmingly backed a motion saying the election result was "at odds with the will of the people".
Prime Minister Viktor Yanukovych was declared the winner, but challenger Viktor Yushchenko has alleged fraud and has called for a re-run.
Parliament cannot overturn the result, but its views may carry weight with the supreme court. The court meets on Monday to examine fraud claims.
Parliament held an emergency session in the capital Kiev to discuss the crisis, and its proceedings were broadcast live on all national TV channels and to thousands of opposition protesters gathered outside the building.
A majority of MPs also passed a vote of no-confidence in the country's Central Election Commission, which like the other vote was not legally binding.
Before the vote, parliamentary speaker Volodymyr Lytvyn told MPs the most realistic solution would be to invalidate the poll.
"The logical issue to be raised is to declare the election politically invalid
because the true will of the people is now impossible to establish," he said.
Dutch Foreign Minister Ben Bot - speaking for the European Union - said the "ideal outcome" would be to hold new elections before the end of the year.
The EU's foreign policy chief, Javier Solana, who has been taking part in mediation efforts in Kiev, has described last Sunday's vote as fraudulent, adding that relations with Ukraine depend on a democratic resolution.
Mr Yushchenko has called on his supporters, who have held massive protests in Kiev all week, to stay on the streets until they achieve "victory".
Rival protests have been staged in the eastern city of Donetsk by supporters of Mr Yanukovych.
Shouting "Yanukovych! Yanukovych!" tens of thousands of people gathered in the city's main square on Saturday. Many of them say the eastern region of Ukraine would rather form an independent republic than accept Mr Yushchenko as head of state.
POLITICAL CRISIS TIMELINE
21 Nov: Viktor Yanukovych declared winner of presidential run-off
Independent observers declare the elections flawed, and thousands of people take to the streets
25 Nov: Supreme court suspends publication of election result until it considers the opposition's complaints
26 Nov: Mr Yanukovych and Mr Yushchenko hold talks and agree to seek peaceful solution
27 Nov: Parliament backs resolution declaring the election invalid, passes vote of no-confidence in the election commission
The BBC's Jonathan Charles in Donetsk says the support for Mr Yanukovych is not surprising - he used to be the regional governor and is credited with improving salaries and pensions.
Mr Yanukovych and Mr Yushchenko held talks on Friday, but agreed only to set up a working group to discuss a peaceful solution.
Independent observers, including some from the EU, have said the elections were flawed.
Mr Yushchenko's party wants a repeat vote to be held on 12 December, under the supervision of the Organisation of Security and Co-operation in Europe.
Mr Yanukovych has accused the opposition, which has been blocking access to government buildings, of attempting to stage a coup.
According to the official election result, the pro-Russian Mr Yanukovych won with 49.46% of the vote against Mr Yushchenko's 46.61%.
The supreme court suspended the presidential poll result on Thursday to consider the opposition's complaints.