DONETSK, Ukraine — Driven out of their key stronghold in eastern Ukraine, pro-Russia separatists regrouped Sunday in the city of Donetsk, vowing to renew their fight against the government in Kiev before thousands of cheering supporters at a rally.

Ukrainian troops on Saturday forced the rebels out of Slovyansk, a city of about 100,000 that had been the center of the fighting. The government’s success there suggested it may finally be making gains in the months-long battle against the insurgency.

President Petro Poroshenko said, while it was not a total victory, purging Slovyansk of the armed insurgents had “incredible symbolic importance.”

It was not yet clear, however, whether the rebels have been permanently crippled and their setback will also increase pressure on Russia to do more to help them.

Rebel fighters from Slovyansk and other towns taken over by the Ukrainian army moved freely Sunday around central Donetsk, a city of 1 million and the capital of a region where the separatists have declared independence in the name of the Donetsk People’s Republic.

The insurgents control the regional administration building and checkpoints on the city outskirts, but Ukrainian forces demonstrated their superior firepower in repelling a rebel attempt to take control of Donetsk airport in late May. That battle left dozens of rebel fighters dead.

Igor Girkin, the defense minister of the separatist republic, told the Russian television channel Life News on Sunday that he would keep up the fight from Donetsk.

“We will continue the combat operations and will try not to make the same mistakes that we made in the past,” said Girkin, a Russian also known by his nom de guerre, Igor Strelkov. Ukrainian authorities have identified him as a veteran of the Russian military intelligence agency.

At the rally Sunday afternoon on a square in Donetsk, people waved flags from Russia and the Donetsk People’s Republic.

“We will begin a real partisan war around the whole perimeter of Donetsk,” Pavel Gubarev, the self-described governor of the Donetsk People’s Republic, told the crowd. “We will drown these wretches in blood.”

But he said the insurgents could easily die in Donetsk if Russia did not come to their aid.

They were forced to flee Slovyansk because several commanders had betrayed Girkin and left his forces vulnerable to attack, Gubarev said.

Despite the bravado in the city, the mood was dire Sunday at a rebel checkpoint on the outskirts of Donetsk.

“We will fight to the end because we have nowhere left to retreat,” said one of the masked fighters, a 32-year-old former coal miner who would give only his first name, Artyom, due to fears of retaliation. “I don’t want to fall into the hands of the Ukrainian authorities, those fascists.”

He said the insurgents still hope for help from Russia “but the hope grows weaker with every day.”

Ukraine and the West have accused Russia of fomenting the insurgency by sending troops and weapons, including tanks and rocket launchers. Moscow has denied providing soldiers or military equipment, saying that Russians fighting in the east are private citizens. The insurgents have shot down Ukrainian aircraft, but the military has a clear superiority in firepower.

Rebel leaders have pleaded with the Kremlin for military assistance, and some prominent Russian nationalists have taunted President Vladimir Putin for failing to send troops to the Russian-speaking area.

Putin has so far resisted, wary of having more Western sanctions slapped on Russia.

Nina Yakovleva, a 45-year-old accountant and resident of Donetsk, said she expected nothing good to come of the convergence of rebels in the city.

“We are afraid that Donetsk will be left in ruins like Slovyansk,” she said. “The rebels have brought us war and fear.”

Pro-Russia insurgents also have been fighting Ukrainian troops in the neighboring Luhansk region, which, like Donetsk, sits along the border with Russia.

The insurgency began in mid-April after the ouster of Ukraine’s pro-Moscow president in February and Russia’s annexation of Crimea in March.