In the deadliest day of Ukraine's protests so far, at least 20 people, including demonstrators and policemen, have been killed, according to reports.
The scuffles with the riot police started in Kiev early on Tuesday morning, soon after thousands of protesters marched towards the parliament, where opposition leaders accused pro-government factions of dragging their feet on a constitutional reform that would limit presidential powers - a key opposition demand.
The confrontation grew into deadly clashes as the police stormed barricades set up by anti-government protesters at the capital's Independence Square and were met with Molotov cocktails.
After shutting down nearby underground metro 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.
About 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. The building of trade unions that served as a base for the protesters for the past two months also caught fire. Emergency crews have been trying to put out a fire for hours and evacuated four trapped women.
"This Satanic government will be destroyed. Death to these scoundrels," Oleh Tyagnibok, one of the three main opposition leaders, told protesters from the stage at Independence Square.
Unrest broke out in Ukraine last year when President Viktor Yanukovich rejected 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.
'Brink of tragedy'
With protests in Kiev showing no sign of ending, Yanukovich has come under mounting Western pressure to strike a deal with the opposition and resolve the 12-week-old crisis.
Speaking to Al Jazeera on Tuesday, Valentin Yakushik, a political science professor at the University of Kiev, said: "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."
As reports trickled in of protesters taking over government buildings in other Ukrainian cities, a meeting at the presidential residence between Yanukovich and opposition leaders Vitaly Klitschko and Arseny Yatsenuk broke down, according to opposition sources.
|Police crack down on Ukraine's Maidan movement
Anna German, a member of parliament, told the Korrespondent.net news website that Yanukovich was planning to give a televised speech early on Wednesday.
"Yanukovich is reacting to the situation absolutely inadequately," Klitschko said in a statement published on the website of his UDAR party.
"All he was talking about was that the opposition leaders call on people at the Maidan to stop resisting and lay down arms."
The Russian Foreign Ministry said that Tuesday's 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".
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 White House said Vice President Joe Biden called on Yanukovich on Tuesday to pull back government forces and exercise maximum restraint.