The European Union is urging Ukraine to scrap new laws that are viewed as curtailing fundamental rights such as freedom of expression and the holding of peaceful protests after a night of vicious streets battles.
The 28-nation bloc's foreign ministers have said the laws rammed through Ukraine's parliament last week under "doubtful procedural circumstances" must be scrapped.
Ukraine has been shaken since November by massive public protests after Russia lured the country's leaders with financial incentives to ditch closer cooperation with the EU.
The laws are widely seen as an attempt to silence the protests but new rallies over the weekend drew tens of thousands of people and turned violent.
Anti-government protesters and police have clashed anew today in the capital Kiev. Hundreds of protesters, many wearing balaclavas, hurled rocks and stun grenades and police responded with tear gas.
The EU statement calls on all parties to "exercise restraint", urging authorities "to fully respect and protect the peaceful demonstrators' right to assembly and speech".
Last night's violence was a sharp escalation of Ukraine's two-month political crisis, which has brought round-the-clock protest gatherings, but had been largely peaceful.
Opposition leader Vitali Klitschko tried to persuade demonstrators to stop their unrest, but failed and was sprayed by a fire extinguisher in the process. Mr Klitschko later travelled to President Viktor Yanukovych's residence and said he had agreed to negotiate.
"There are only two ways for events to develop. The first one is not to negotiate," Mr Klitschko was quoted as saying by the Interfax news agency. "A scenario of force can be unpredictable and I don't rule out the possibility of a civil war. ... And here we are using all possibilities in order to prevent bloodshed."
Mr Yanukovych said later on his website that he had tasked a working group, headed by national security council head Andriy Klyuev, to meet opposition representatives to work out a solution to the crisis.
But it was unclear if either side was prepared for real compromise - throughout the crisis, the opposition has insisted on the government's resignation and calling early presidential elections.
The White House blamed the increased tensions on Ukraine's government for failing to acknowledge its people's legitimate grievances and threatened sanctions if the use of violence continued.
National Security Council spokeswoman Caitlin Hayden said Ukraine's government "has moved to weaken the foundations of Ukraine's democracy by criminalising peaceful protest and stripping civil society and political opponents of key democratic protections under the law".
She called on Ukraine to repeal recent laws limiting protests, remove riot police from central Kiev and start talking to the opposition.
"The US will continue to consider additional steps - including sanctions - in response to the use of violence," she said.
The crisis erupted in November after Mr Yanukovych's decision to freeze ties with the European Union and seek a huge bail-out from Russia. The decision sparked protests which increased in size and determination after police twice violently dispersed demonstrators.
But anger rose substantially after Mr Yanukovych last week signed an array of laws severely limiting protests and banning the wearing of helmets and gas masks.
Many of yesterday's demonstrators wore hard hats and masks in defiance of the new laws, set several police buses on fire and some of them chased and attacked officers.