Clashes have taken place after large crowds of pro-EU demonstrators rallied in Ukraine's capital Kiev against new laws restricting public protests.
Stun grenades and flares were thrown as protesters tried to reach parliament, their way blocked by police and buses.
Opposition politician Vitali Klitschko later said President Viktor Yanukovych had agreed to set up a cross-party commission to resolve the crisis.
The US appealed for an end to the violence and urgent political talks.
Mr Klitschko, who leads the Udar party, made comments after meeting President Yanukovych at his residence outside Kiev.
"The president pledged to create on Monday morning a commission with representatives from the presidential administration, cabinet and opposition to find a solution to the crisis situation," Udar quoted the former world heavyweight boxing champion as saying.
Arseniy Yatseniuk, another opposition leader, said Mr Yanukovych called him to say that he was ready for talks.
The new legislation was passed with a quick show of hands on Thursday by MPs loyal to the president, who then signed it into law.
The opposition accused the ruling party of a coup.
US and EU officials have expressed deep concern at the new legislation.
Ukraine's current anti-government movement began in protest at Mr Yanukovych's decision in late November to pull out of a landmark treaty with the EU, but has expanded to demand his resignation.
'Extremists and provocateurs'
Sunday's rally in Kiev, attended by tens of thousands, heard calls from opposition politicians to disregard the new laws curbing protests that pro-EU demonstrators have been staging for the past two months.
Clashes erupted as some people headed away from the main square towards parliament, to vent their anger over the new laws. They ran into police cordons near Kiev's Dynamo football stadium, some 300 metres from Independence Square.
They pelted police with flares, thunder flashes and petrol bombs, the BBC's Daniel Sandford in Kiev reports.
They overturned a bus used by riot police and interior troops and set it alight. Other vehicles were also set on fire.
The police could be seen behind buses sheltering under their riot shields, and occasionally throwing their own thunder flashes and gas canisters to try to force the crowd back, our correspondent says.
Police also used water cannon to try to disperse the demonstrators.
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Well, this is rather silly now, you'd think it'd be obvious for a protest to happen against a law that decides for you what you CAN and CAN'T do during a protest, because no one would really care whether or not they can throw a bomb or not, right?