Geneva: The United Nations human rights office said Monday it was deeply troubled by the arrest of Alexei Navalny, and called for the Russian opposition leader's immediate release.
In a tweet, UN human rights chief Michelle Bachelet's office reiterated a call for an impartial investigation into his poisoning in August, with what Western experts concluded was the Soviet-designed nerve agent Novichok.
"We are deeply troubled by the arrest of Alexei Navalny, and call for his immediate release and for his due process rights to be respected in line with the rule of law. We reiterate our call for a thorough and impartial investigation into his poisoning," the tweet said.
Navalny, 44, is the most prominent opponent of Russian President Vladimir Putin. He was arrested at a Moscow airport after returning from Germany where he had been recovering from the poisoning.
Russia's FSIN prison service said it had detained Navalny for violating the terms of a suspended sentence he was given in 2014, on fraud charges he says were politically motivated.
Back in September, Bachelet had called on Moscow to conduct or cooperate with a "thorough, transparent, independent and impartial investigation".
"The number of cases of poisoning, or other forms of targeted assassination, of current or former Russian citizens, either within Russia itself or on foreign soil, over the past two decades is profoundly disturbing," the former Chilean president had said.
She also criticised the failure in many cases to hold perpetrators accountable and provide justice for the victims or their families.
"Navalny was clearly someone who needed state protection, even if he was a political thorn in the side of the government.
"It is not good enough to simply deny he was poisoned."