Ukraine's Moscow-backed Prime Minister Viktor Yanukovych has confirmed he will contest the re-run of the disputed presidential poll on 26 December.
Yanukovych supporters voiced their anger at the ruling
He spoke a day after the Supreme Court ruled that last month's run-off, which gave him victory over opposition leader Viktor Yushchenko, had been fraudulent.
Mr Yanukovych angrily attacked the ruling, saying it was unconstitutional.
Parliament, meanwhile, failed to adopt electoral law changes the opposition said were needed to prevent fraud.
The emergency session was adjourned until 14 December without passing a package of bills that would reshuffle the central electoral commission and make other key changes to ensure fair balloting in the repeat run-off.
Ukraine's Socialists and Communists had promised to vote for the amendments if the opposition supported a constitutional reform aimed at trimming presidential powers.
But the deal collapsed after pro-Yushchenko lawmakers said they would only consider the constitutional changes after the electoral amendments were approved.
The central election commission met on Saturday to fix 26 December as the date for the new poll, a decision which needs parliament's and President Leonid Kuchma's approval.
"I am certain the Supreme Court's decision is a violation of the Ukrainian constitution and that it was taken under pressure from the street," Mr Yanukovych said in a statement given to journalists.
"Without any doubt I have no other choice but to run again and to win," he said.
The BBC's Stephen Mulvey reports from Kiev that there had been speculation that the outgoing president was considering backing another candidate with popular appeal outside the east of the country.
Mr Yanukovych's supporters were angry at the ruling and vowed to vote for him again.
At a meeting in the eastern city of Kharkiv on Saturday, officials from Ukraine's eastern and southern regions - where Mr Yanukovych enjoys strong support - urged both Mr Yanukovych and Mr Yushchenko to withdraw from the contest.
The meeting also discussed calls for greater regional autonomy.
The original 21 November run-off had been criticised by Western observers over what they said were numerous irregularities.
The prime minister and President Kuchma had pressed for a completely new election. Mr Kuchma has yet to issue his response to the court ruling.
But Mr Yushchenko wanted - and got - a re-run of the second round only.
Thousands of opposition protesters celebrated through the night after the verdict, which came after nearly a fortnight of street demonstrations in the capital.