The men belong to a group known as the Activists of Kulikovo Field, which had camped out on the square of that name in Odessa. They had been in jail for two days after being rescued from the trade union building blaze that killed at least 40 other pro-Russian militants.

After establishing a tent camp on Kulikovo Square, the activists, following the example of pro-Russian groups in Donetsk in eastern Ukraine, had proclaimed an independent state, the Republic of Novorossiya. An exile in Moscow then claimed the presidency.

But unlike in eastern Ukraine, they faced strong opposition from pro-Ukrainian activists, including soccer fans. The fan clubs of the Chernomorets and Metalist teams, who had a long and fierce rivalry that often led to fights, recently declared a truce because of the crisis. Before a match on Friday, they scheduled a march in support of Ukrainian unity down Deribasovskaya Street, a pedestrian thoroughfare in the heart of the city.

The fighting started when the Activists of Kulikovo Field tried to block the march. Leaflets went up around Odessa on Friday morning, announcing the pro-Russian action to “defend Odessa from pogroms,” and the city braced for violence.

What followed were hours of bloody street clashes involving bats, pistols and firebombs. Many in the pro-Russian group carried bowie knives, photographs show. Four people died on and around Deribasovskaya Street. The pro-Russians, outnumbered by the Ukrainians, fell back, abandoning their tent camp on Kulikovo Square, which was burned by their opponents. Many then sought refuge in the trade union building.

Yanus Milteynus, a 42-year-old construction worker and pro-Russian activist, said he watched from the roof as the pro-Ukrainian crowd threw firebombs into the building’s lower windows, while those inside feared being beaten to death by the crowd if they tried to flee.

“Some people jumped or tried to run away, but they chased them and beat them,” he said. Videos of the inferno, however, also show pro-Ukrainian activists trying to move scaffolding from a stage to the building, to rescue those inside.