Protesters clashed with riot police in Kyiv on Sunday, as up to 100,000 Ukrainians massed in defiance of new laws aimed at stamping out anti-government demonstrations.
A group of radical activists began attacking police with sticks as protesters tried to push toward the Ukrainian parliament building, which has been cordoned off by rows of police and buses.
The protesters, many wearing hard hats and gas masks, used stun grenades, fire extinguishers and flares as they attacked police in riot gear.
Former world boxing champion Vitaly Klitschko, a key opposition leader who spoke at the main rally, tried to stop protesters from attacking police but was shouted down.
Earlier at the main rally, Klitschko denounced the new laws, which ban nearly all forms of protests and were rushed through parliament last week by supporters of President Viktor Yanukovych.
"One hundred twenty MPs from the Party of the Regions raised their hands in the parliament and the authorities declared that the Ukrainian people and their rights are illegal. This will not succeed, we will not let it happen this way."
The new laws permit the arrest of protesters who wear masks or helmets and allow the authorities to jail up to five years those who blockade public buildings.
Other provisions ban the dissemination of "slander" on the Internet and introduce the term "foreign agent" to be applied to non-governmental groups that receive foreign funding.
The United States and other Western governments have denounced the new laws as anti-democratic.
Anti-government protesters have filled the streets of Kyiv and other Ukrainian cities since November when President Yanukovych reversed his position and refused to sign a trade deal with the European Union, opting instead for stronger ties with Russia.
Since then, Moscow has promised to buy $15 billion in Ukrainian debt and cut the price of Russian gas to Ukraine to strengthen the struggling economy.