Kremlin critic Alexei Navalny was detained by Russian police at passport control after flying back to Russia from Germany for the first time since being poisoned last summer.
He flew from Berlin to Moscow, arriving in Russia for the first time since he was nearly killed by a nerve agent attack.
The activist says the authorities were behind the attempt on his life, an allegation backed up by investigative journalists but denied by the Kremlin.
Russia's FSIN prison authority confirmed that officers had detained him, the Interfax news agency reported.
Metal barriers were erected inside the airport, Vnukovo, and Russian media reported that several activists - including key Navalny ally Lyubov Sobol - had been detained there.
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.
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.
Warned of imprisonment
The Russian authorities had warned 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.
Navalny has asserted that Mr Putin is doing all he can to stop his opponent from coming back by fabricating new cases against him.
What happened to Navalny?
In August, the opposition leader collapsed on a plane flying home from Tomsk in Siberia to Moscow and the pilot diverted the flight to the city of Omsk, from where he was eventually allowed to fly on to Germany in an induced coma.
He was released from hospital in Berlin in September to continue his recuperation.
Navalny said recently he was able to do push-ups and squat exercises, and therefore had probably almost fully recovered.
Last month, investigative reporters named three FSB agents who had travelled to Tomsk at the time Mr Navalny was there, and said the specialist unit had tailed him for years.
Navalny then, in a phone call, duped an FSB agent named Konstantin Kudryavtsev into revealing details of the operation against him, according to the Bellingcat investigative group.
The agent told him that the Novichok used to poison him was placed in his underpants.
Mr Kudryavtsev said during the phone call he had been sent to Omsk later to seize Mr Navalny's clothes and remove all traces of Novichok from them.
President Putin has dismissed the investigation by Bellingcat and others into who poisoned Mr Navalny as "a trick" and said that he was backed by US intelligence services.