An emergency meeting between Ukraine's government and opposition leaders was scheduled for Monday after clashes between pro-West demonstrators and police turned Kiev into a battleground.
As many as 100,000 people gathered in sub-zero conditions Sunday on Independence Square, known as the Maidan, in defiance of new laws that the U.S. and other Western governments say effectively outlaw peaceful protests.
The laws come after hundreds of thousands of people took to the streets in Kiev in December against the former Soviet republic's plans to shift ties away from the European Union and toward Vladimir Putin's Russia.
Vitali Klitschko, the former boxing star who is the opposition movement's most prominent voice, called for peaceful protest on Sunday. However, a smaller group numbering in the hundreds splintered from the main throng and headed toward the parliament building, Reuters reported.
A video showed Klitschko addressing protesters using a megaphone, only for someone in the crowd to spray him with a fire extinguisher.
Equipped with helmets, shields, masks and clubs, and some displaying Far Right insignia, the smaller group hurled fireworks and Molotov cocktails and torched several police buses.
Video footage showed the masked group clash in hand-to-hand fighting with police and interior ministry troops.
According to the BBC and Reuters, the interior ministry said at least 30 police officers were injured and 10 hospitalized. Pictures showed troops responding with water cannons and demonstrators reported they were fired on with rubber bullets.
Klitschko told a local TV station later on Sunday that he "didn't rule out the possibility of a civil war," according to the report by the BBC.
Close to midnight, Ukraine President Viktor Yanukovych issued a statement saying a working group of Cabinet and presidential representatives would meet opposition leaders Monday.
The meeting would "consider the issues regarding the settlement of political crisis," the statement posted on the president’s English-language website said.
The U.S. said Sunday it was "deeply concerned" by the violence, but said the clashes were a "direct consequence of the government failing to acknowledge the legitimate grievances of its people."
The statement, which was released by National Security Council spokeswoman Caitlin Hayden, added that the Ukraine had weakened its democracy and "criminalized peaceful protest." The statement added the the U.S. had not ruled out imposing sanctions.
During December's protests, Republican Sen. John McCain and Democratic Sen. Chris Murphy traveled to Kiev to give their support to the opposition. "To all Ukrainians, America stands with you," McCain told the crowd.
Hollywood star George Clooney also posted a video last month in which he told the demonstrators: "When you look to the West, know that we are looking back at you with great admiration."
Reuters contributed to this report.