By Maria Tsvetkova

DONETSK, Ukraine (Reuters) - Pro-Russian separatists on Sunday seized control of the offices of regional state television in the eastern Ukrainian city of Donetsk and said they would take it off air and broadcast a Kremlin-backed Russian channel instead.

A Reuters reporter said four separatists in masks, with truncheons and shields, were standing at the entrance to the building controlling access, while more separatists in camouflage fatigues could be seen inside.

About 15 police officers were standing a short distance away but were not trying to resist the separatists. One police lieutenant, who was sitting in a police vehicle nearby, said it would have been pointless to intervene.

It was the first time the station had been seized by the separatists, though previously a transmission tower in the Donetsk region had briefly been seized and technicians forced to broadcast Russian stations' output.

Pro-Russian separatists, some of them armed, have seized about a dozen official buildings in eastern Ukraine. They say they are rising up against a Ukrainian government they say is illegitimate, but Kiev says they are proxies of the Russian government bent on destabilizing Ukraine.

About an hour after the station in Donetsk was overrun, it was still broadcasting its scheduled programs, a children's show called "Circle of the Sun".

CHANGING CHANNELS

But the station's director, Oleg Dzholos, who came outside to speak to reporters, said the people who seized the building had ordered him to change the programming.

"They used force to push back the gates," he said. "There were no threats. There were not many of my people. What can a few people do? The leaders of this movement just gave me an ultimatum that one of the Russian channels has to be broadcast."

Dzholos said three of his staff were still inside the building, and that the separatists had not ejected him from his office.

Separatists who swear allegiance to the self-proclaimed Donetsk People's Republic already control the regional governor's office and the city hall in Donetsk, the regional capital.

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A man in a white shirt who came out of the building and said he was a representative of the Donetsk People's Republic said that from now on the station would be broadcasting Rossiya 24, a Russian state-owned news channel.

Earlier, a crowd of around 400 people surrounded the building and shouted "Russia!" and "Referendum!," a reference to a vote the separatists want to hold on seceding from Ukraine. The protesters later drifted away, but the separatist guards on the doors remained.

One of the masked men at the entrance, asked why the building had been seized, said: "They show lies, they try to influence the people and they broadcast misinformation."

The police officer sitting in his vehicle nearby, who gave his name as Vitaly, said his superiors had ordered him to protect the building after they received information that a crowd was heading to the television station.

"I don't see any point in using force," he said. "It would not have worked if we had tried to stop anybody, there were a lot of people here."

(Reporting by Maria Tsvetkova; Writing by Christian Lowe; Editing by Nigel Stephenson)