Tear gas filled the air in Kiev as riot police and masked youths clashed over the passage of a new law aimed at curbing public protests.
The Kyiv Post said it appeared the violence in the area around European Square was spontaneous rather than orchestrated by protest leaders. The instigators were described by the newspaper as young people who were tired of waiting for the government to back down.
The Post said protesters threw cobblestones and set a police bus on fire. The two sides remained squared off as nightfall approached but were not physically engaged.
Opposition leaders, including heavyweight boxing champ Vitali Klitschko, appealed for calm but the crowd appeared to be in no mood to listen, the Post said.
Speakers at the rally included a former Ukraine admiral who called on the armed forced to ignore "illegal" orders from the government, the BBC said.
Kiev has been rocked by large, noisy protests since November over the government's intentions to strengthen its economic and political ties with neighboring Russia rather than with the western nations of the European Union.
The government responded last week by passing a law clamping new restrictions on speech assembly, which protesters said violated the Ukraine constitution.