In the deadliest day of Ukraine's protests so far, at least 13 people, including seven demonstrators and six policemen, have been killed, according to reports.
The violence erupted in the centre of the capital Kiev on Tuesday when police stormed barricades set up by anti-government protesters and were met with Molotov cocktails.
After shutting down nearby subway stations and restricting traffic, police armed with stun grenades and water cannons moved into Independence Square, which has been the centre of nearly three months of protests, and dismantled some of the barricades.
The 20,000 demonstrators fought back, armed with rocks, bats and fire bombs, and singing the Ukrainian national anthem as the main protest camp was engulfed in flames.
Police said six officers were killed in the clashes, and Olha Bilyk, spokeswoman for the Kiev city police, told the Associated Press news agency that seven civilians died, including three who were shot.
|Police crack down on Ukraine's Maidan movement
Unrest broke out in Ukraine last year when President Viktor Yanukovich spurned a free-trade agreement with the European Union and opted for a $15bn package of Russian credits and cheaper gas to support Ukraine's ailing economy in November.
The rioting on Tuesday began after opposition leaders accused pro-government factions in parliament of dragging their feet on a constitutional reform that would limit presidential powers - a key opposition demand.
Within hours of the clashes, the Russian Foreign Ministry said that the violence was a "direct result of connivance by Western politicians and European structures that have shut their eyes ... on the aggressive actions of radical forces".
In a statement, he urged the Ukrainian opposition to "refrain from threats and ultimatums and establish a substantive dialogue with the authorities with the aim of extricating the country from deep crisis".
Thousands of protesters had filled Independence Square earlier in the day, sensing that the political crisis was reaching a critical turning point.
The opposition leader, Vitali Klitschko, called on women and children to leave the camp over fears of a possible police assault.
Denouncing the "grave new escalation" in Kiev, EU foreign policy chief Catherine Ashton condemned "all use of violence, including against public or party buildings".
"I urge the leadership of Ukraine to address the root causes of the crisis," she said, calling for an urgent return to a parliamentary process.
Echoing Ashton's sentiments a number of Western countries, including the US and France, called for an end to the violence in Kiev and restraint by security forces.
The renewed clashes have piled more pressure on Yanukovich to strike a deal with the opposition and defuse the 12-week-old crisis.
"Today is the new stage of the events in Ukraine. We are coming to a stage which we may call being at the brink of tragedy," Valentin Yakushik, a political science professor at the University of Kiev, told Al Jazeera.
"Now the official opposition cannot control the people protesting in the streets. They came to the tactics of vandalism, burning down some buildings, destroying cars, throwing stones, molotov cocktails. Very dangerous situation."