Liberal challenger Viktor Yushchenko told supporters after meeting the western envoys and Prime Minister Viktor Yanukovich, who won according to the official count, that only a new vote could redeem the election he says was rigged.
The debate opened at noon (1000 GMT), with the agenda focusing on the disruption caused by mass protests over the election outcome.
The postponement of Yanukovich's inauguration after a supreme court ruling was also due to be discussed.
Parliament has no power to overturn last Sunday's poll after the central election commission declared Yanukovich the winner.
But it could criticise the commission, and its assessment could carry political weight two days before a supreme court case examining Yushchenko's complaints of poll irregularities.
"We are counting on parliament to give its political assessment of the central election commission," Yushchenko told a vast crowd assembled in Kiev's Independence Square for the fifth day running.
All groups backed the sitting, except Yanukovich's Regions of Ukraine party. A similar debate this week was closed amid pandemonium as Yushchenko symbolically took the oath of office.
Thousands swayed to live music in the square overnight after Yushchenko told them to remain until all was won.
"My order, my request, my prayer to you is this: nobody must leave this square until victory," he said.
Thousands remain on the streets
in support of the opposition
Friday's talks, bringing together mediators from Russia and the European Union, produced little more than an agreement to set up a group to pursue talks. Both hopefuls also pledged to shun violence and allow the government to keep functioning.
Yushchenko emerged to say he had rejected Yanukovich's proposal to submit 11,000 complaints to the courts.
"We will only hold talks on staging a new vote," he said, adding he sought an election on 12 December.
European Union foreign policy chief Javier Solana said the idea of another election would be discussed when the working group met on Saturday. "Without doubt, a third election is a possibility," he said.
Yanukovich said nothing as he left the meeting.
It was unclear which side gained more.
Russia, which had urged Ukrainians to settle their differences in the courts, offered no comment.
The dispute has increased tension between the West and Russia, which had backed Yanukovich and his notion of boosting ties with Moscow.
Yushchenko describes Russia as a strategic partner, but seeks gradual integration with western Europe.
Current PM Viktor Yanukovich (L)
enjoys the backing of Moscow
Western countries have bluntly criticised the official results. The European Union and the United States said the vote fell far short of international standards.
Prime Minister Yanukovich had earlier addressed a crowd of supporters by Kiev's railway station, mostly young men brought from mines in his native Donbass coalfield in the Russian-speaking east.
Organised into tight units of about 100, most carried large sticks. Many were clearly drunk.
"Dear friends, together we must do everything so that an unconstitutional coup in Ukraine does not happen," Yanukovich said.