Kremlin critic Alexei Navalny is due back in Moscow later on Sunday for the first time since he was nearly killed by a nerve agent attack last year.
He was seen on board a plane that left Berlin shortly after 14:30 GMT.
The activist says the authorities were behind the attempt on his life, an allegation backed up by investigative journalists but denied by the Kremlin.
Mr Navalny, 44, faces arrest on his return from Germany. Extra police have been deployed at the airport in Moscow.
Metal barriers have been erected inside the airport, Vnukovo, and Russian media are reporting that several activists – including key Navalny ally Lyubov Sobol – have been detained there.
Mr Navalny’s spokeswoman Kira Yarmysh posted on social media pictures of police cars at the airport.
«Никому не интересный блогер» возвращается в Россию. Заборы в аэропорту, автозаки, запрет на вход в здание без билета, люди, согнанные встречать Бузову (кому вообще в голову пришла эта безумная идея?!) pic.twitter.com/XGidV0BUnV
— Кира Ярмыш (@Kira_Yarmysh) January 17, 2021
Mr Navalny – who had been treated in Germany – earlier appealed to supporters to meet him off the flight, and a “Let’s meet Navalny” page has been set up on Facebook (in Russian). Thousands of people have said they will go or expressed an interest, despite forecasts of extreme cold.
After boarding his Pobeda airlines plane, the opposition politician said: “I’m sure that everything will be absolutely fine, I’m very happy today.”
He also accused the Kremlin of encouraging people to go to Vnukovo to see a pop star, Olga Buzova, in a bid to squeeze out his supporters.
Mr Navalny collapsed on an internal flight in Siberia last August, and it later emerged he had been poisoned with a Novichok nerve agent.