Ukrainian authorities must conduct a swift, full and transparent investigation into Sunday’s arson attack on nationwide broadcaster Inter TV, the International Press Institute (IPI) said today.

Demonstrators reportedly hurled incendiary devices into Inter’s news offices in Kiev on Sunday afternoon, damaging the building and leading several people inside to be treated for smoke inhalation and another for a broken leg.

The demonstrators, approximately 20 individuals described in some reports as former military servicemen, had gathered to protest what they said was Inter’s pro-Russian editorial line.

Ukrainian President Petro Poroshenko described the attack as “unacceptable”, a reaction IPI shared.

“Regardless of objections to news or editorial content, violence is never appropriate,” IPI Director of Advocacy and Communications Steven M. Ellis said. “People have the right to protest against content they disagree with – to meet speech with speech – but this attack unnecessarily put people at risk and it both overshadows and undermines the message protestors ostensibly sought to advance. We urge authorities to bring those responsible to justice and demonstrate to those who would consider such an attack in the future that it will not be tolerated.”

The nature of the attack was not immediately clear. Some reports indicated that at least one incendiary device had been thrown into the building. However, Inter released a statement in which it alleged that some 15 of the protestors forced their way into the building, fired shots and set it ablaze.