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 is reporting that at least one activist has been held there.
Mr Navalny's press-secretary Kira Yarmysh posted on social media pictures of police cars at the airport.
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.
Russian authorities have consistently denied any role in the poisoning, and the Kremlin has rejected Mr Navalny's claims that President Vladimir Putin himself ordered it.
The Putin critic has said he misses Moscow, is almost fully recovered from the attack, and that there was never any doubt he would return.
The Russian authorities have warned Mr Navalny could face imprisonment after missing a prison service deadline in December to report at an office in Moscow.
The prison service accuses him of violating conditions imposed after a conviction for embezzlement, for which he received a suspended sentence. He has always condemned the case as politically motivated.
Separately, Russia's investigative committee has launched a new criminal case against him on fraud charges related to transfers of money to various NGOs, including his Anti-Corruption Foundation.
Mr Navalny has asserted that Mr Putin is doing all he can to stop his opponent from coming back by fabricating new cases against him.
News media from around the world gathered at Berlin airport to record the activist's departure from Germany - but Russian federal TV channels and news agencies are ignoring his return.
Source: BBC