Thousands of Ukrainians have massed in central Kiev to protest against alleged fraud in parliamentary elections won by the ruling party, and leaders of the opposition threatened not to recognise the new legislature.
At least 2,000 opposition supporters carrying Ukrainian flags gathered outside the headquarters of the central election commission on Monday amid a heavy presence of elite anti-riot police.
Political tensions have surged in Ukraine as the authorities have still failed to publish final results from the October 28 elections, more than a week after voting finished.
Commentators expect the ruling Regions party of president Viktor Yanukovych to take a wafer-thin majority, despite a strong challenge from the opposition led by jailed former premier Yulia Tymoshenko.
Tymoshenko's Fatherland party said in a statement it was "ready to declare" the new parliament invalid unless the authorities "stopped the falsification of the electoral process".
The threat has been backed by other main opposition parties, the nationalist Svoboda (Freedom) movement and the UDAR party of boxer Vitali Klitschko.
'Splitting the choice'
Arseniy Yatsenyuk, who is heading the Fatherland coalition in the absence of the jailed Tymoshenko, accused the Regions party of "stealing the votes of Ukrainians and changing the results in favour of its candidates".
The firebrand leader of Svoboda, Oleg Tyagnybok, meanwhile said that the authorities were "spitting on the choice of Ukrainians."
The opposition's claims are focused on the vote count in a dozen or so districts where they say that the vote was falsified in favour of Regions.
A small change in the number of seats won could make a huge difference, as the Regions party is expected to only find a slim majority with the help of loyal independent legislators.
Tyagnybok vowed that the protest would be "open-ended," even though a court had earlier declared any protest action illegal in central Kiev until November 12.
Asked by reporters about fears that the police would disperse the protest, Yatsenyuk replied: "Let them try to send the Berkut with their truncheons," referring to the anti-riot police.
The European Union's top diplomat Catherine Ashton and EU commissioner for enlargement Stefan Fule had expressed alarm on Saturday about the failure to publish final results so long after voting took place.
"Complaints should be dealt with swiftly and in full respect of rules and established procedures," the statement added.
OSCE observers have already expressed concern that the elections were a step backwards for Ukraine, a country seen as one of the most democratic in the former Soviet Union after the 2004 Orange Revolution uprising.
The observers argued that the election was skewed from the start by the jailing of Tymoshenko in an abuse of power case that the West fears is an act of revenge on the part of Yanukovych against his old political rival.