Taras Stetskyv, a member of parliament and long-standing Ukrainian nationalist, told a crowd massed on Kiev's Independence Square that "the authorities - Kuchma and Yanukovich - used the talks to cheat".
Another Ukrainian opposition leader, Mykola Tomenko, added that if parliament failed to use its "last opportunity" to dismiss the government, political means would be exhausted and resistance would begin to grow.
"I am not authorised to say what we will do if [parliament] fails. God help us, to take this decision."
His statements raised the temperature in a dispute which shows signs of erupting into civil conflict.
On Tuesday, protesters broke through the main gates of the country's parliament in Kiev which has been surrounded for the last few days.
Aljazeera's correspondent Akram Khuzam reported the situation was dangerous and that the authorities may announce a state of emergency.
Prime Minister Viktor Yanukovich was the official winner of last week's election to replace outgoing President Leonid Kuchma.
President Kuchma's preferred
successor is Yanukovich
But facing massive pressure from the Western governments who have rejected the result, the prime minister has offered a re-election and even suggested both he and Yushchenko step down if election rigging is proven.
Talks between Yushchenko's camp and supporters of Yanukovich have made little progress since mediators from the European Union and Russia secured agreement on the creation of a working group to resolve the crisis.
Parliament failed on Tuesday to proceed with a vote of no confidence in Yanukovich's government and a new session of the assembly was scheduled for Wednesday.
The speaker of parliament in neighbouring Russia, meanwhile, warned that Ukraine was heading either for a split or for bloodshed, and blamed opposition protesters for failing to keep a promise to clear the streets.
"What is happening in Ukraine saddens us," said Duma Speaker Boris Gryzlov in Moscow. "The situation is heading towards a split or towards bloodshed."
Yanukovich, who favours continuing close ties with neighbouring Russia, was Kuchma's preferred successor, and also had tacit but potent support from Moscow.
His strongest support lies in the mainly Russian-speaking eastern half of Ukraine.
Yushchenko, himself a former prime minister under Kuchma, has in contrast said he would focus more on developing stronger relations with the West.
His base of support lies mainly in the Ukrainian-speaking west of the country and among younger people.