Amnesty International has urged the Ukrainian government to investigate the violent assault from lawmakers of the far-right Svoboda (Freedom) Party on Oleksandr Panteleymonov, head of the First National TV Channel.

Dramatic footage showed at least five ultranationalists, including Igor Miroshnichenko, beating Panteleymonov over the public broadcasting of Vladimir Putin and pro-Russian Crimea leaders signing the annexation treaty.

Panteleymonov was accused of serving Putin and being "Moscow trash". The mob then forced him to sign a letter of resignation.

Ironically, Miroshnichenko is a member of the parliamentary Committee on freedom of speech.

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

The London-based human rights group also reported that the thugs abducted Panteleymonov telling him that he would be forced to apologise to protesters in Kiev's Maidan. He was freed a few hours after the attack came out.

"This assault on freedom of expression must be immediately investigated and those responsible must be prosecuted. Anything less will only open the door to further attacks against media professionals and activists," said Heather McGill.

"The acting Ukrainian authorities must waste no time in demonstrating that basic human rights are protected in Ukraine and that nobody will face discrimination because of their political views or ethnic origin."

The European Broadcasting Union has slammed the attack, saying in a statement:

On behalf of the EBU and all the public service broadcasters of Europe, we condemn this vicious attack on the director general of NTU. Such actions are totally unacceptable, and we call on the Ukrainian authorities to react swiftly and resolutely. Independent public service media and freedom of expression are indispensable to a proper democracy.

The attack was also criticised by prime minister Arseniy Yatseniuk, Svoboda's political ally, who said:

"These are not our methods. The actions of these lawmakers are unacceptable,"

Even Svoboda party head and Miroshnichenko's friend Oleh Tyagnybok condemned the violent incident. "Such actions were fine yesterday (during the protests), but now they are inappropriate," Tyahnybok said in official statement.

The attack sparked a protest by journalists of First National Channel:

Journalists @ Ukraine's AG office in Kyiv demand prosecution of far-right MPs after attack on @1tvua CEO via @skorykm pic.twitter.com/4MCvgSYQqQ — Maxim Eristavi (@MaximEristavi) March 19, 2014

National Television Company (NTU) is state-run and operates Ukraine's largest public broadcaster, First National Channel.

During the Maidan anti-government protests, the First National Channel was seen as supportive of then president Viktor Yanukovich and biased in its coverage.

Opposition leader Vitaly Klitschko has demanded to remove the Svoboda MPs behind the attack and put them on trial.