MARIUPOL, Ukraine — Thousands of steelworkers fanned out on Thursday through the city of Mariupol, establishing control over the streets and banishing the pro-Kremlin militants who until recently had seemed to be consolidating their grip on power, dealing a setback to Russia and possibly reversing the momentum in eastern Ukraine.

By late Thursday, miners and steelworkers had deployed in at least five cities, including the regional capital, Donetsk. They had not, however, become the dominant force there that they were in Mariupol, the region’s second-largest city and the site last week of a bloody confrontation between Ukrainian troops and pro-Russian militants.

While it was still far too early to say the tide had turned in eastern Ukraine, the day’s events were a blow to separatists who recently seized control here and in a dozen or so other cities and who held a referendum on independence on Sunday. Backed by the Russian propaganda machine and by 40,000 Russian troops just over the border, their grip on power seemed to be tightening every day.

But polls had indicated that a strong majority of eastern Ukrainians supported unity, though few were prepared to say so publicly in the face of armed pro-Russian militants. When President Vladimir V. Putin of Russia withdrew support for the separatists last week, calling for a delay in the referendum and for dialogue on Ukraine’s future, the political winds shifted, providing an opening that the country’s canny oligarchs could exploit.