BRUSSELS — 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, 44, was returning home from 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".

Taking to Twitter, Charles Michel, the president of the European Council, issued a demand for Russian authorities to release Navalny immediately, branding the move to detain him as "unacceptable".

Other EU countries have also added their voice to calls for the politician to be freed. France expressed "great concern" at the news and urged for his immediate release.

"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.

Navalny had defied warnings from Russian authorities in returning home, particularly from Russia's prison service which had said that he faced immediate arrest upon his return to the country.

Dozens of Navalny's supporters, journalists and riot police units were waiting for him at Vnukovo airport.

His plane landed at Moscow's Sheremetyevo airport from Berlin after being diverted from its planned destination, according to Euronews correspondent Galina Polonskaya who was on board.

She said Navalny seemed relaxed during the plane journey and was joking with reporters during the flight.

He refused to give any interviews on the plane and when someone shouted to him asking if he was afraid of being arrested he laughed and replied: "Me, arrested? Why should I be arrested?"

The pilot of the plane first announced a delay of 30 minutes before indicating that the plane was heading for Sheremetyevo and would not land at Vnukovo airport as scheduled due to a "technical problem", Polonskaya said.

Navalny told his team that he had not expected this to happen, adding: "This shows to what extent they are afraid of me."

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.

Security measures at Vnukovo airport were heightened on Sunday, with several prisoner-transport trucks parked outside. — Euronews