Britain on Wednesday imposed sanctions against six leaders of the Polar Wolf colony, where Russian opposition leader Alexei Navalny died last week, Reuters reports.

Western leaders expressed outrage at news from Russian authorities that Navalny, President Vladimir Putin’s most prominent domestic critic, had collapsed and died in prison.

According to British Foreign Secretary David Cameron, the sanctioned individuals, including the warden and five deputy wardens of the penitentiary, will be banned from entering Britain and their assets will be frozen.

“It is clear that the Russian authorities saw Navalny as a threat and repeatedly tried to silence him… no one should doubt the repressive nature of the Russian system,” he said.

“Those guilty of the brutal treatment of Navalny should have no illusions – we will bring them to justice.”

The Russian embassy in London condemned the measures as “unjust” and said Britain was interfering in Russia’s internal affairs and passing judgments until the investigation into Navalny’s death was complete.

“We consider London’s decision to be legally senseless and invalid,” Russian news agencies note. “Like all previous anti-Russian provocations, it will not go unanswered.”

Cameron vowed to take action on Saturday, while London summoned the Russian ambassador to make it clear to Russian authorities that they were “fully responsible” for Navalny’s death.