Ukraine's opposition threatened to go on the attack on Thursday unless the government agreed to concessions to quell protests that have left five activists dead in the first fatalities of two months of anti-government rallies.
Protesters and police were locked in a tense standoff early Thursday after ferocious clashes that turned an area of central Kiev into a virtual war zone, with police using tear gas, stun grenades and rubber bullets against protesters hurling stones and Molotov cocktails.
The bloody clashes marked a new peak in tensions after two months of protests over the government's failure to sign a deal for closer integration with the European Union under Russian pressure.
Opposition leaders including former world boxing champion Vitali Klitshcko have launched talks with President Viktor Yanukovych and they were due to meet again Thursday, with the chief demand of early elections.
The leader of the opposition Fatherland party Arseniy Yatsenyuk had warned the protesters on Independence Square Wednesday evening that Yanukovych had 24 hours to agree a peaceful solution, saying he was ready to die for the cause.
"If he does not go down that path then we will go forwards together and if it means a bullet to the head, then it is a bullet to the head.
"Viktor Yanukovych you have 24 hours. Take a decision. I have taken my decision," he said to cheers.
Klitschko told crowds on Independence Square after talks on Wednesday that protesters will go "on the attack" if Yanukovych does not swiftly offer concessions.
"If Yanukovych does not make concessions, then tomorrow (Thursday) we will go on the attack," Klitschko said, adding the president could resolve the situation without bloodshed by giving way to the chief demand of early elections.
Oleg Musiy, the coordinator of the medical service, told pro-opposition Hromadske radio, that five people have been killed and around 300 wounded in Wednesday's clashes.
According to the Ukrainska Pravda news website, four of the five people found dead had gunshot wounds.
Ukrainian Berkut riot police were still holding their line on Grushevsky Street in central Kiev as hundreds of protesters faced them on the other side of barricades, an AFP correspondent said.
There were sporadic clashes with demonstrators throwing stones and the police responding with stun grenades but there was a relative lull compared with Wednesday's ferocious fighting.
The demonstrators had overnight further fortified their barricades with sandbags filled with snow.
Their frontline was marked by burning tyres which were still on fire in blazes that the police had been unable to extinguish despite the use of water cannon. The tyres sent a rancid plume of black smoke billowing into the Kiev sky.
Meanwhile tens of thousands Wednesday evening filled Independence Square in Kiev which was the main protest hub for the last two months, hoping their sheer numbers would deter any attempt by police to disperse the rally.
They sought to reinforce the protest barricades by several metres by filling sandbags with snow and brought new tyres for burning on the frontline.
Police had on Wednesday upped the stakes in the protests by launching a major assault, initially pushing demonstrators back well beyond their initial lines by marching forward in military formation.
But the protesters then regrouped, creating a semi-circle of new barricades out of burning black tyres whose flames burned in near-apocalyptic scenes.
A former MP from Yanukovych's Region's Party who switched sides during the protest, Inna Bogoslovska, bluntly told the rally on Independence Square the authorities were doomed: "Viktor Fyodorovich, this is the end," she said.
The deadly violence horrified Ukrainians, who have never witnessed such scenes in their country including during the 2004 Orange Revolution which was almost entirely peaceful.
Amid calls for sanctions against the Ukrainian government, European Commission president Jose Manuel Barroso on Wednesday warned the authorities that the EU executive authority would assess "possible actions" against the Ukrainian authorities.
The United States also revoked the visas of several Ukrainian nationals linked to violence against protesters in November and December last year, the US embassy said in a statement.
But Russia, which has regarded Ukraine's pro-EU protest movement with suspicion from the start, took a different view and blamed the opposition and West for the clashes.
"Ukraine's legitimate authorities face outside interference in its internal affairs," Russian Deputy Foreign Minister Grigory Karasin told the Interfax news agency, referring to EU and US statements.
But President Vladimir Putin's spokesman Dmitry Peskov said Thursday that Russia will not intervene in the protests and believes Ukraine's leadership will find a way out.
"We consider we do not have the right to intervene in any way in the internal affairs of our brother Ukraine. That's unacceptable and Russia has not done this and will not do it," Peskov said in an interview published on the website of Komsomolskaya Pravda daily.