KIEV, Ukraine — It is by now a well-established pattern. Armed, masked men in their 20s to 40s storm a public building of high symbolic value in a city somewhere in eastern Ukraine, evict anyone still there, seize weapons and ammunition, throw up barricades and proclaim themselves the rulers of a “people’s republic.” It is not clear who is in charge or how the militias are organized.

Through such tactics, a few thousand pro-Russian militants have seized buildings in about a dozen cities, effectively establishing control over much of an industrial region of about 6.5 million nestled against the Russian border.

Day by day, in the areas surrounding the cities of Donetsk and Luhansk, pro-Russian forces have defied all efforts by the central government to re-establish its authority, and on Wednesday, Ukraine’s acting president conceded what had long been obvious: The government’s police and security officials had lost control.

“Inactivity, helplessness and even criminal betrayal” plague the security forces, the acting leader, Oleksandr V. Turchynov, told a meeting of regional governors in Kiev. “It is hard to accept but it’s the truth. The majority of law enforcers in the east are incapable of performing their duties.”