Putin critic Alexei Navalny has been detained after flying back to Moscow from Germany, five months after he was nearly killed by a nerve agent attack.
The activist, 44, was led away by police at passport control at Sheremetyevo airport.
Thousands had gathered at a different Moscow airport to greet his flight from Berlin, but the plane was diverted.
Mr Navalny blames Russian authorities for the attempt on his life last year. The Kremlin denies any role.
The opposition politician's allegations have however been backed up by reports from investigative journalists.
News of Mr Navalny's detention prompted condemnation from the European Union, France and Italy, which called for his immediate release.
US President-elect Joe Biden's incoming national security adviser echoed the call. "The Kremlin's attacks on Mr Navalny are not just a violation of human rights, but an affront to the Russian people who want their voices heard," Jake Sullivan said.
How did events unfold?
When Mr Navalny was poisoned last August and collapsed on an internal flight in Siberia, he was flown to Germany for emergency medical treatment. As he recovered, he said he intended to return to Russia.
On Sunday he made good on that pledge, boarding a Pobeda Airlines flight despite warnings he would face arrest on landing.
The plane was packed with journalists, including Andrey Kozenko of the BBC Russian Service. Shortly before landing, the pilot announced that for "technical reasons", the plane was being diverted from Vnukovo airport to Sheremetyevo airport, causing a stir among the passengers.
Why was Navalny detained?
In a statement late on Sunday, Russia's prison service said the opposition leader "had been wanted since 29 December 2020 for repeated violations of the probation period". It added that he would remain in custody until a court decision.
The authorities accuse him of violating conditions imposed after a conviction for embezzlement, for which he received a suspended sentence. He has always said the case was politically motivated. Read the news here. : slotxo