Controversial anti-protest laws which sparked unprecedented riots in Ukraine entered force on Tuesday as the latest standoff between thousands of protesters and anti-riot police in Kiev moved into a third day.
The new laws, which ban nearly all forms of protest in the ex-Soviet country, were officially published in the newspaper of the Ukranian parliament after a warning from President Viktor Yanukovych that the violence threatened the entire country.
They allow for jail terms of up to five years for those who blockade public buildings and the arrest of protesters wearing masks or helmets.
Other provisions ban the dissemination of "slander" on the Internet.
The move to bring them into force came despite calls from the West and the opposition to bin the legislation, raising fears that the authorities could use the restrictions to resort to violence to disperse the protest.
Clashes on Sunday and Monday, which followed two months of protests, turned the centre of the capital Kiev into a veritable war zone as some 10,000 demonstrators battled security forces.
Fireworks and stun grenades lit up the night sky while the deafening drumming of protesters with sticks on metal echoed through the streets.
The violence in a country where the pro-democracy 'Orange revolution' in 2004 peacefully overturned a rigged presidential poll and forced a new ballot is unprecedented.
The clashes erupted after a rally of some 200,000 people against the restrictions on protesting was pushed through by Yanukovych supporters in parliament on Sunday.
In a televised address to the nation, Yanukovych warned on Monday that the violence threatened the foundations of the entire country, which is divided between the pro-European west and the pro-Russian east.
"I am convinced that such phenomena are a threat not only to the public in Kiev but all of Ukraine," he said, indicating his patience was wearing thin.
"I treated your participation in mass rallies with understanding, I expressed readiness to find ways to solve the existing contradictions."
The opposition led by three politicians including former world boxing champion Vitali Klitschko said it was ready for dialogue but stressed it wanted to hold talks with Yanukovych, not his aides.
The government set up a special commission to address the crisis.
Ukraine's Prosecutor General Viktor Pshonka warned protesters to halt "mass rioting", describing it as a crime against the state.
Protests began after Yanukovych's refusal to sign a pact for closer integration with the EU in November.
With more than 200 people injured so far, thousands of Ukrainians braved temperatures of minus 10 degrees Celsius (14 degrees Fahrenheit) to take part in the standoff with police.
According to the Kiev health authorities, more than 100 protesters were wounded in the violence.
Health officials said three people lost eyes and one person had his hand amputated, health officials said.
The interior ministry said more than 100 members of the security forces had been wounded.
The ministry added that several dozen people had been arrested for mass rioting.
In the epicentre of the clashes outside the entrance to the iconic Dynamo Kiev football stadium in central Kiev, both sides hunkered down behind barricades on Monday.
Protesters lobbed stones dug up from the cobbled road, flung Molotov cocktails and threw fireworks over a 20-metre (65-foot) no-man's land at police lines.
Police responded by throwing stun grenades and occasionally using rubber bullets and tear gas, while the most radical protesters used lasers to blind security forces.
"Who, if not us, and when, if not now," read a banner carried by one group of protesters.
Opposition leaders, including Klitschko and Arseniy Yatsenyuk, appeared unable to have any influence on the hard core of radical protesters and stopped short of supporting their actions.
But Ukraine's jailed former prime minister and opposition leader Yulia Tymoshenko came out in support of those clashing with police, saying she would be with them if she could.
"Protect Ukraine and do not fear anything," she said. "You are heroes."
The White House urged an end to the violence, with US National Security Council spokeswoman Caitlin Hayden warning that Washington was still considering sanctions against Ukrainian officials.
EU foreign ministers meeting in Brussels on Monday deplored the violence, saying the government was at fault for adopting the repressive laws.
The curbs on protests were "the most solid package of repressive laws that I have seen enacted by a European parliament in decades," Swedish Foreign Minister Carl Bildt said in Brussels.
It was not clear who was behind the radicalisation of the protest, which appeared to have been a well-organised move.
Ukrainian media linked the action to a hitherto little-known right-wing youth group called "Right Sector".