The US and several European governments have called for the immediate release of leading Kremlin critic Alexei Navalny after he was detained at a Moscow airport on Sunday.
The opposition figure was returning home from a convalescence in Germany five months after an attempt on his life in a nerve agent poisoning he blames on Russian authorities.
President Vladimir Putin’s government has denied responsibility for the attack.
The US and the EU led calls on Monday for Putin’s fiercest critic to be released, but have stopped short of threatening retaliatory action.
US Secretary of State Mike Pompeo said he was “deeply troubled” at the news of Navalny’s arrest.
In a thinly-veiled rebuke of the Russian government, he added that “confident political leaders do not fear nor see the need to commit violence against or wrongfully detain, political opponents”.
“Together with its European partners, it is following his situation with the utmost vigilance and calls for his immediate release,” the French foreign ministry said in a statement.
In a post on Twitter, Canada’s foreign minister Marc Garneau said Navalny’s detention was “unacceptable,” adding that “Canada strongly condemns” his arrest.
“Russian authorities must immediately release him,” he said.
“We will continue to demand an explanation into his poisoning,” Garneau added.
The Russian opposition leader announced on Wednesday that he would return to his homeland after recovering from being poisoned with a nerve agent, despite Russian authorities’ threats to put him behind bars again.
Navalny, who has blamed his poisoning in August on the Kremlin, said that Putin was now trying to deter him from coming home with new legal motions.
Moscow has repeatedly denied a role in the opposition leader’s poisoning.
Last month, Navalny released the recording of a phone call he said he made to a man he described as an alleged member of a group of officers of the Federal Security Service (FSB) who he purports poisoned him in August and then tried to cover it up. The FSB dismissed the recording as fake.