Leonid Kravchuk, president from 1991 to 1994, urged parliament to "act with the greatest responsibility" as it began a debate on an amnesty for detained protesters.
He said: "The state is on the brink of civil war. We must call what is happening by its proper name. What is happening is revolution because we are talking about an attempt to bring about a change of power."
His comments came as opposition deputies and supporters of President Viktor Yanukovich Yanukovich were in talks over the wording of a draft law under which protesters detained so far by police would get amnesties.
Though the unrest began because of Mr Yanukovich's U-turn on closer ties with Europe it has since turned into a mass demonstration, punctuated by violent clashes between protesters and police, against perceived misrule and corruption under Mr Yanukovich's leadership.
Several hundred people camp round-the-clock on Kiev's Independence Square and along an adjoining thoroughfare, while more radical activists confront police lines at the nearby Dynamo football stadium.
Anti-Yanukovich activists have also stormed into municipal buildings in many other cities across the sprawling country of 46 million people. Hundreds of protesters in Kiev have occupied City Hall and the main agricultural ministry building.
Opposition leaders, including boxer-turned-politician Vitaly Klitschko, were resisting demands by Mr Yanukovich's Regions Party for barricades to be removed from roads and for protesters to leave occupied buildings as a pre-condition for an amnesty for detained activists.
Mr Klitschko, in a comment which also highlighted the tenuous control the opposition leaders have over sections of the protest movement, said: "For us to simply say to people 'you have done your job, now go home' is now not possible."
In a big concession to the opposition and the protest movement, pro-Yanukovich deputies voted on Tuesday to repeal a series of sweeping anti-protest laws which they brought in hastily earlier this month in response to increasingly violent clashes.
Prime Minister Mykola Azarov resigned on the same day in an attempt to appease the protesters and the opposition, and though his deputy has taken over as acting Prime Minister it is unclear when a new government will be formed.
With Mr Azarov, a Russian-born hardliner, gone there had been speculation Moscow might slow or even halt the stream of aid agreed when Ukraine abandoned closer links with the EU.
But yesterday Russian President Vladimir Putin ordered his government to honour the multibillion-pound bailout deal for Ukraine.
Mr Putin said the aid would come even if the opposition forms the next government in Kiev.
He added: "I would ask the government to fulfill all our financial agreements in full."
However, his Economy Minister Alexei Ulyukayev cast doubt on the timing of the instalment.
He said: "Our commitment to fulfilling these obligations has been confirmed. As for the schedule and parameters, this is an issue that requires further discussion with our Ukrainian colleagues and consideration of the restructuring of the government."