At least 42 people were killed in a street battle between supporters and opponents of Russia in southern Ukraine that ended with dozens of pro-Russian protesters incinerated in a burning building, bringing the country closer to war.
The riot in the Black Sea port of Odessa, ending in a deadly blaze in a besieged trade union building, was by far the worst incident in Ukraine since a February uprising that ended with Kremlin-backed president Viktor Yanukovych fleeing the country.
Events took a violent turn on Friday when a column of soccer supporters, chanting support for Ukraine's leaders, clashed with men in black, some firing pistols. Television images showed police caught between the two sides.
Clashes then spread along the streets until rebels moved into a large trade union building.
Police said four people were killed and dozens wounded in the clashes, and at least 37 people died in the blaze. On Saturday, police raised the death toll in the city to 42, easily the largest number of casualties since about 100 people were killed in the Euromaidan protests that toppled Yanukovych in February.
Kiev's Interior Ministry blamed pro-Russian protesters for the incident, saying they had attacked the pro-Ukrainians before retreating to the trade union headquarters, from where they opened fire on the crowd and threw out the petrol bombs that caused the blaze.
Kiev said the violence was provoked by foreign demonstrators sent in from the self-proclaimed Transdnestr republic, where Moscow has a military garrison. It said most of the dead who had been identified so far were from there.
"Today we Ukrainians are constantly being pushed into confrontation, into civil conflict, toward the destruction of our country to its heart. We cannot allow this to happen," said acting President Oleksander Turchinov.
But the Kremlin, which has massed tens of thousands of soldiers on Ukraine's eastern border and proclaims the right to invade to protect Russian speakers, blamed the government and Western countries for the deaths.
"Kiev and its Western sponsors are practically provoking the bloodshed and bear direct responsibility for it," RIA Novosti quoted Kremlin spokesman Dmitry Peskov as telling reporters.
On Saturday morning, people placed flowers near the burnt-out doors of the trade union building, lighting candles and putting up the yellow, white and red flag of the city. About 2,000 pro-Russian protesters outside the burnt-out building chanted "Odessa is a Russian city."
The Odessa clashes spread the violence from the eastern separatist heartland to an area far from the Russian frontier, raising the prospect of unrest sweeping more broadly across a country of around 45 million people the size of France.
A couple of hundred pro-Russian protesters in the eastern city of Donetsk stormed the governor's business premises and the state security headquarters, ransacking files and smashing windows. The attack reflected growing disorder in the area, targeting as it did a security building that had already been brought under rebel control.
"This is for yesterday!" said Tatiana Kamniva outside the governor's office. "They're monsters, worse than monsters."
Regional police chief Petro Lutsiuk said Saturday more than 130 people had been detained and could face charges ranging from participating in riots to premeditated murder.
"We are not stopping"
The Odessa bloodshed came on the day that Kiev launched its biggest push yet to reassert its control over separatist areas in the east, hundreds of kilometers away, where armed pro-Russian rebels have proclaimed a "People's Republic of Donetsk."
On Saturday the government said it was pressing on with the offensive in the area for a second day, and had recaptured a television tower and a security services building from rebels in Kramatorsk, a town near the rebel stronghold of Slovyansk. Authorities said six people were killed in fighting.
"We are not stopping," Interior Minister Arsen Avakov said in a post on Facebook.
The Donetsk region administration said snipers were shooting from rooftops in Kramatorsk, shops were closing and there was an atmosphere of panic. A Reuters correspondent reported in late afternoon, however, that the town was quiet.
Vasyl Krutov, head of a government "anti-terrorist center" behind the operation in the east, told a news conference: "What we are facing in the Donetsk region and in the eastern regions is not just some kind of short-lived uprising, it is in fact a war."
Donetsk is planning to hold a referendum on May 11 on secession from Ukraine, similar to one staged in March in Ukraine's Crimea region, which was seized and annexed by Russia in a move that overturned the post-Cold War diplomatic order.