New violence erupted in Ukraine's capital during the night as a large crowd attacked a government exposition and conference hall where police were stationed inside.

Early Sunday, demonstrators were throwing firebombs into the Ukrainian House building and setting off fireworks, and police responded with tear gas. Although the crowd created a corridor at the building's entrance apparently for police to leave, none were seen coming out.

The outburst underlined a growing inclination for radical actions in the protest movement that has gripped Kyiv for two months. The building under attack is about 250 metres down the street from Independence Square, where mostly peaceful demonstrations have been held around the clock since early December and where protesters have set up an extensive tent camp.

Clashes with police broke out a week ago in the wake of new, harsh anti-protest laws.

More-moderate opposition figures, including heavyweight boxing champion Vitali Klitschko, have tried appealing to stop the clashes, but have been booed, and one time Klitschko was sprayed with a fire-extinguisher.

Another top opposition figure, Arseniy Yatsenyuk, said Saturday that the protests will continue despite the offer by embattled President Viktor Yanukovych to make him prime minister.

The protests began in Kyiv after Yanukovych shelved a long-awaited trade pact with the European Union in favour of securing a bailout loan from Russia, and boiled over into violence a week ago over the new anti-protest laws.

'Tuesday is Judgment Day'

Yatsenyuk told the crowd at the main protest site that Yanukovych must still meet several key demands of the opposition and that talks will continue.

Yatsenyuk said a special session of parliament called for Tuesday could be decisive. Yanukovych has said that session could discuss a government reshuffle and changes to the new anti-protest laws.

Those laws ignited clashes between police and protesters a week ago at a site near the Ukrainian parliament, about a 10-minute walk from Independence Square.

The building under attack on Sunday lies between those two points. The assault started after an estimated 200 police were seen entering the building and speculation spread that they were preparing to disperse demonstrators.

"Tuesday is judgment day," Yatsenyuk told a large crowd of protesters on Independence Square. "We do not believe a single word of theirs. We believe only actions and results."

At a later news conference, Yatsenyuk said "we are not throwing out the proposal, but we are not accepting it, either. We are conducting serious consultations among three opposition forces."

He also said the opposition would demand to sign a free trade agreement with the European Union and free political prisoners, including former Prime Minister Yulia Tymoshenko.

Early elections

The opposition also is demanding early presidential elections. Another top protest leader, Vitali Klitschko, told the crowd that they would press ahead with that demand.

About an hour after the opposition leaders spoke, demonstrators began attacking a building about 100 metres from the square where police were stationed, smashing windows, breaking doors and hurling firebombs.

A protester throws a Molotov cocktail towards riot police in Kyiv. More protests are expected to crop up around the country. (Efrem Lukatsky/Associated Press)

Yanukovych's latest offer, coming as protester anger rises and spreads from the capital to a wide swath of the country, appeared to have been both a concession and an adroit strategy to put the opposition in a bind.

Accepting the offer could have tarred Yatsenyuk among protesters as a sell-out, but rejecting it would make him appear obdurate and unwilling to seek a way out of the crisis short of getting everything the opposition wants.

The offer came hours after the head of the country's police, widely despised by the opposition, claimed protesters had seized and tortured two policemen before releasing them. The opposition denied it and said Interior Minister Vitali Zakharchenko was making a bogus claim in order to justify a police sweep against protesters.

Three protesters have died in the past week's clashes, two of them from gunshot wounds and a third of unspecified injuries. The Interior Ministry said a policeman was found shot in the head overnight. No arrests have been made or suspects named.

Protesters have rained stones and firebombs on police while officers retaliate with stun grenades and tear gas.

In the meeting with opposition leaders where he made the offer to Yatsenyuk, Yanukovych also agreed to discuss ways of changing Ukraine's constitution toward a parliamentary-presidential republic, which was one of the demands of the opposition, according to a statement on the presidential website.

If that change went through, the prime minister would have more powers and would be elected by parliament, not appointed by the president. Yanukovych backers currently have a majority in the parliament and the next election for the legislature will be in 2017.

'We will be obliged to use force'

Earlier, Zakharchenko said the two police officers were released with the help of negotiations by foreign embassies. He said they had been hospitalized, but didn't give details of how they allegedly were abused. He earlier said the officers were seized by volunteer security guards at the protest gatherings in Kyiv and held in the city hall, which protesters have occupied since December and turned into a makeshift dormitory and operations centre.

But the commandant of the corps, Mykhailo Blavatsky, told The Associated Press that no police had been seized.

"The authorities are looking for a pretext to break up the Maidan and creating all kinds of provocations," he said. "Capturing a policeman would only give the authorities reason to go on the attack and we don't need that."

Anti-government protesters, right, clash with interior ministry members at the regional administration headquarters in the town of Vinnytsia, some 200 kilometres southwest of Kyiv. (Reuters)

Zakharchenko earlier said a third captured officer had been released and was in serious condition in a hospital.

"We will consider those who remain on the Maidan and in captured buildings to be extremist groups. In the event that danger arises, and radicals go into action, we will be obliged to use force," Zakharchenko said.

In Lviv, where support for Yanukovych is minuscule, regional lawmakers on Saturday voted to establish a parallel government. Although the move was largely symbolic, it demonstrated the strong animosity toward the government in Ukraine's west. Ukrainian politics largely is divided between the Russian-speaking east, which is the industrial heartland, and the Ukrainian-speaking west.

On Saturday, about 100 protesters briefly occupied the headquarters of the energy ministry in downtown Kyiv. Minister Eduard Stavitskiy said the country's nuclear energy facilities were placed on high alert