A lawmaker from Ukraine's far-right, ultra-nationalist party has been filmed assaulting the head of state television in his own office before forcing him to write a letter of resignation.

Igor Miroshnichenko visited the offices of First National TV on Tuesday evening and accused channel boss Oleksandr Panteleymonov of working for the Russian authorities after the station broadcast Vladimir Putin's speech on Crimea.

A video of the incident was posted online and the resignation letter was published on Facebook by the Svoboda Party's Press Secretary Aleksandr Aronets.

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"You lied to Ukrainians using their own money," Miroshnichenko shouts in the video. (The physical assault comes about 4:40 into the video below).

Miroshnichenko is a member of parliament for the far-right Svoboda (Freedom) Party - which has been described by some analysts as "neo-fascist." He is the deputy head on Ukraine's freedom of speech committee. He was accompanied by at least five other people.

Miroshnichenko is not a new member of parliament, he was a lawmaker during the administration of former President Viktor Yanukovych. But since Yanukovych's ouster, the right-wing Svoboda Party has gained more ministry positions, now making up around a quarter of the ministries including that of defense minister. The Right Sector, an even more extremist group which made up the most militant aspects of the Kiev protests last month, also has stronger links to the new government.

Panteleymonov's letter asked for him to be released from his position and said he was writing it “on his own free will.” As of Wednesday, it was not clear if he had formally left his position.

"It is astonishing that a member of the parliamentary committee on freedom of speech was involved in this attack," said a statement by Heather McGill, Ukraine researcher at rights group Amnesty International. "The acting authorities must send a signal that this sort of behavior will not be tolerated in Ukraine."

Russia has legitimized its actions in Crimea by saying it is protecting ethnic Russians from the "fascist" government in Kiev. The West has largely dismissed this as propaganda, but the incident at First National TV highlights the fact that a sizable proportion of the country's new ministers are aligned with the extreme right.

- NBC News' Alexander Smith