Dozens of angry men hurled rocks, smashed the windows and broke into a police station in the city of Horlivka not far from the border with Russia, and flew the Russian flag from the building.
Hundreds of onlookers outside chanted "Referendum!" and "Russia!" A video posted online showed a policeman confronting a separatist, then being beaten by the mob and taken away in an ambulance. Other policemen switched their allegiance to the demonstrators.
Later in the day, armed men in masks also seized control of a military airport outside the city Slovyansk, also in the Donetsk region.
The developments came as European Union foreign ministers met in Luxembourg to consider further sanctions against Russia, and three days ahead of a scheduled conference in Geneva involving diplomats from the United States, Russia, the European Union, Ukraine and Switzerland, which is intended to seek ways of defusing tensions.
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One of the men directing the raid in Horlivka introduced himself as a lieutenant colonel of the Russian army to a line of policemen who switched sides, but he did not state his full name. The man, dressed in green camouflage uniform, instructed them to obey their new chief and to attach St George's ribbons to their uniforms, which have become a symbol of pro-Russian protesters in eastern Ukraine.
The episode fed into accusations from the government in Kiev and Western officials that the protests are being instigated by the Kremlin and that some of the insurgents are Russian agents.
"The people came to tell him that he is a puppet of the Kiev junta and they won't accept him," Sapunov said.
Acting Deputy Interior Minister Mykola Velichkovych acknowledged today that some police officers in eastern regions were switching sides. "In the east we have seen numerous facts of sabotage from the side of police," Velichkovych told reporters.