Russian opposition figure Alexei Navalny, who has spent five months in Berlin recovering from a poisoning attack, was on a plane headed for Moscow on Sunday, AFP journalists on board reported.
Navalny's flight on budget airline Pobeda took off from Berlin's BER airport at 2.17pm GMT. Navalny risks being arrested on arrival and potentially jailed for years, but he told reporters on board that he was "an innocent person".
The plane is scheduled to land at Moscow's Vnukovo airport around 7.20pm (4.20pm GMT).
Navalny, one of President Vladimir Putin's most prominent domestic critics, announced his decision to return from Germany on Wednesday, saying he missed Moscow and was not interested in what he called new fabricated criminal cases against him.
A day later, the Russian capital's prison service said it would do everything to arrest him once he returned, accusing him of flouting the terms of a suspended prison sentence for embezzlement, a 2014 case he says was trumped up.
Navalny, who is hoping for success in parliamentary elections in September, faces potential trouble in three other criminal cases too, all of which he says are politically motivated.
Analysis: Despite arrest risk, Navalny must return to Russia to retain political credibility
His return poses a conundrum for the Kremlin: jail him and risk protests and punitive Western action by turning him into a political martyr. Or do nothing and risk looking weak in the eyes of Kremlin hardliners.
Navalny, 44, departed from Berlin where he was flown in August for emergency medical treatment after being poisoned with what German tests showed was a Novichok nerve agent.
The opposition politician, who says he has nearly fully recovered, says Putin was behind his poisoning. The Kremlin denies involvement, says it has seen no evidence that he was poisoned, and that he is free to return to Russia.
Navalny says the Kremlin is afraid of him. The Kremlin, which only refers to him as the "Berlin patient", laughs that off. Putin allies point to opinion polls that show the Russian leader is far more popular than Navalny, whom they call a blogger rather than a politician.
Supporters plan to greet Navalny
Navalny's supporters plan to meet him on arrival at the airport despite a forecast of bitterly cold minus 17C weather and over 4,500 new coronavirus cases a day in the Russian capital.
So far, at least 2,000 people have used a Facebook page to say they plan to be there, with another 6,000 expressing an interest. Pro-Kremlin activists are also expected to turn up.
The Moscow prosecutor's office, which says it has officially warned 15 pro-Navalny organisers, has said the event is illegal because it is not sanctioned by the authorities. That means that people who turn up could be detained, fined or jailed.
Citing Covid-19 restrictions, the airport has said it will not allow media inside.
A Moscow court on Saturday ordered a Navalny ally, Pavel Zelensky, to be held in pre-trial detention on extremism charges, which he denies.
On the eve of his return, Navalny took to Facebook to thank Germans for what he described as their friendly hospitality in the last five months.
"Thank you, friends!" he wrote in German.
Russian investigators on Tuesday opened a criminal probe into Kremlin critic Alexei Navalny, alleging he used more than 356 million rubles ($4.8 million) of donations to his organisations for personal purposes including holidays abroad.
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