Ukrainian President Viktor Yanukovych on Saturday offered sweeping concessions for the opposition, including the prime minister's post, but his opponents vowed to press on with protests until all their demands are met as fresh clashes erupted.

Stick-wielding protesters in helmets threw Molotov cocktails and smashed in the windows of a Soviet-era building in Kiev used as a temporary base by security forces in the early hours of Sunday.

Troops holed up inside defended themselves with their riot shields and hurled stun grenades as some of the 2,000 militants clambered over sandbags into the former Lenin Museum shouting "Shame!".

The protesters later withdrew and laid siege to the building, which is next to Independence Square -- the epicentre of a movement that began two months ago with protests against Yanukovych's last-minute decision to revoke a European Union deal.

Officials say three people have been killed in the escalating protests in Kiev over the past week, raising fears of a wider civil conflict as protests have spread to outlying regions of Ukraine.

The opposition says six people have died.

Yanukovych on Saturday offered top opposition leaders Arseniy Yatsenyuk and Vitali Klitschko the posts of prime minister and deputy prime minister in a proposed compromise aimed at ending the former Soviet republic's worst political crisis since independence in 1991.

Speaking to tens of thousands packed into Independence Square, the main opposition leaders were careful to neither explicitly reject nor accept the deal.

"We are not scared of responsibility for the future of Ukraine. We take responsibility and are ready to take the country into the European Union," Yatsenyuk told the cheering crowd.

He added: "We do not believe any one of their words. We believe in action."

Yatsenyuk later told reporters the opposition was "not over the moon" about Yanukovych's unprecedented proposals.

"We are not refusing the offer but we are not accepting it either," he said.

World boxing champion Klitschko, leader of the UDAR (Punch) party, told protesters that the opposition would press for a presidential election due in 2015 to be brought forward to this year.

"Talks will continue," said Klitschko, who along with the other opposition leaders earlier held the latest in a series of negotiations with Yanukovych.

'No confidence' in Yanukovych

The Ukrainian presidency said after Saturday's talks that the two sides had agreed that the protests and the police presence in Kiev would be gradually scaled down, but there was no sign of an easing of tensions on Independence Square.

"I don't think Yanukovych's proposal will change the situation," said Miron Kotsuba, a driver from the western city of Lviv.

"People will not accept the result of negotiations without Yanukovych's resignation. There is no confidence in these authorities," he said.

Iryna Pavlenko, a student from Kiev, said: "It is absurd when the authorities give us a condition for going out of the crisis after all that they have done."

"It is terrible to think of military actions but my friends are in decisive mood," she said.

Yanukovych also said he was willing to consider constitutional changes that would reduce the huge powers he has built up and return to a political system that gives the prime minister more authority.

The president agreed to another key demand of the opposition, saying that he would put a bill to parliament to amnesty those arrested during two months of protests and political crisis in Ukraine.

He also promised to re-consider draconian anti-protest laws passed by parliament in an unprecedently broad series of offers unseen in the weeks of stand-off.

'Spiral of violence'

The protests first erupted in response to Yanukovych's refusal to sign a key deal with the European Union in November.

But they have snowballed into anti-government protests against Yanukovych's four-year rule, which the opposition claims has been riddled with corruption and nepotism.

The authorities have also faced mounting pressure outside Kiev with protesters storming regional administration offices, not just in the anti-Yanukovych west of the country but also north and east of Kiev.

Interior Minister Vitaliy Zakharchenko has bluntly warned that the use of force was a possibility and accused the mainstream opposition of failing to control radicals.

"The events of the last days in the Ukrainian capital have shown that our attempts to solve the conflict peacefully, without recourse to a confrontation of force, remain futile," he said.

In a sign of a possible split within the ruling Regions Party over how to deal with the crisis, however, Ukraine's richest man Rinat Akhmetov said that dialogue was the only way forward.

"There can be only one solution to the political crisis -- a peaceful one. Any use of force is unacceptable," said Akhmetov, an ally of Yanukovych and bankroller of his party.