A clash Friday between supporters of Ukrainian unity and Russian-backed separatists in the Black Sea port of Odessa left 42 people dead and scores injured, police said.
Thirty of the victims died of smoke inhalation after a fire was set in the central trade union building, where pro-Russia separatists reportedly had taken up sniper positions to fire on pro-unity demonstrators, police said.
Eight others died after leaping from upper-floor windows of the burning building, authorities said.
The deadly fire followed a fierce battle between rival groups of marchers in the historic port that straddles Ukraine's volatile ethnic fault line. Odessa is a predominantly Russian-speaking city that is geographically and culturally closer to the pro-Europe western areas of Ukraine.
About 1,500 supporters of the embattled government in Kiev, the Ukrainian capital, held a march Friday evening to denounce attempts by pro-Russia militants to wrest control of Ukrainian territory in the country's east and south.
A rival group of several hundred people, demanding an independence referendum, converged with the unity supporters, prompting volleys of Molotov cocktails, stun grenades and cobblestones, news agency reports from Odessa said.
The street clash left four dead, three from bullet wounds, and 15 wounded, police said. Russia Today television said 37 were wounded in the confrontation. The report by the Kremlin-controlled state television channel also said that video from the scene showed most of those subsequently found dead in the trade union building wore insignia attesting to their support for Russia.
The station also said that the pro-unity demonstrators were joined by ultranationalists and "football fans" who turned the peaceful march into a deadly clash.
On Ukrainian television, however, Odessa lawmaker Dmytro Spivak laid blame on the separatist faction, Reuters news agency reported.
"It is abundantly clear that the pro-Russia side was very well armed, well organized and that this action was planned long ago," Spivak said.
Six young supporters of the Kiev government had been killed in the Odessa
fighting, he said, the deadliest in more than two months of unrest in eastern and southern Ukraine.
The country has been racked by unrest since November, when then-President Viktor Yanukovich unilaterally abandoned an association agreement between Ukraine and the European Union and pledged to strengthen ties with Russia.
A three-month rebellion led to the ouster of Yanukovich in late February, and opposition politicians took up interim control of the government after a majority of Yanukovich's parliamentary allies defected from his Party of Regions or fled the capital.
Separatists led by Russian troops then took control of the Crimean peninsula, occupying the local parliament, which called a referendum on independence. Voters approved separation from Kiev and annexation to Russia, a move that Western powers do not recognize.
Russian President Vladimir Putin and his Kremlin lieutenants contend that the interim leadership in Kiev is unelected and illegitimate and they have defended Russia's land grab in Crimea as necessary to protect its ethnic Russian population.
The Kiev authorities accuse Moscow of fomenting the unrest in eastern Ukraine to disrupt the May 25 presidential election, aimed at restoring legitimate authority in the capital.