Ukraine's new government was facing a stern test of resolve after a nationalist Ukrainian MP physically assaulted the head of a television station in Kiev and forced him to write a resignation letter.

A group of at least five men, including MP Igor Miroshnichenko from the nationalist Svoboda party, barged into Oleksandr Panteleymonov's office, then shouted at him and landed blows before forcing him to write the letter. They filmed the attack and posted it online. The video has received hundreds of thousands of hits.

The men accused Panteleymonov of broadcasting pro-Russian images during the stand-off in Kiev in recent months, and called him "Moscow trash", using a derogatory term for Russians.

Miroshnichenko is a member of the new parliamentary committee on freedom of speech. He said afterwards he did not believe he had done anything wrong but was prepared to be investigated. He added Panteleymonov had "served Putin and Russian propaganda … at a time when Ukrainians were dying on the streets".

Panteleymonov heads the Ukrainian state television company, which runs the First Channel, accused of censorship and biased reporting under the rule of deposed president Viktor Yanukovych. The Svoboda MP and his associates were furious that the channel had transmitted footage from Putin's speech in the Kremlin on Tuesday in which the Russian president announced that Crimea would become part of the Russian Federation.

"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, Ukraine researcher at Amnesty International.

"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."

Russian officials have repeatedly claimed that the new interim government in Kiev is run by radicals and "neo-Nazis", and the actions by the Svoboda MP will play into their hands.

Politician Vitali Klitschko, who plans to stand in the presidential elections in May, said it was important the MPs were punished, calling their acts "savagery and lawlessness". He added: "Ukrainians changed the power in the country because of lawlessness and lack of justice. This should not remain unpunished. All Svoboda MPs who resorted to force should vacate their seats."

Klitschko said that the prosecutor general, who is also from Svoboda, should launch an open and transparent investigation into the incident. "Society expects legal assessment of the excesses of MPs and holding them accountable. Ukraine does not need political looters," he said.

The interior minister, Arsen Avakov, wrote on his Facebook page that while he found the broadcasting of the First Channel to be unacceptable, he found Svoboda's methods to be the same.

"As the interior minister, I am ready to organise all the required investigative efforts as soon as instructed by the general prosecutor," he wrote.

Given that the prosecutor general is also a member of Svoboda, the incident could prove a key test for Ukraine's fledgling government, cobbled together in the aftermath of Yanukovych's flight from Kiev.