Russian opposition leader Alexei Navalny was detained after flying back to Moscow from Germany, five months after he was nearly killed by a nerve agent attack.
The police at passport control at Sheremetyevo airport whisked away the 44-year-old.
Thousands had gathered at a different Moscow airport to greet his flight from Berlin, but the plane was diverted.
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.
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.
“I know that I’m right. I fear nothing,” Mr Navalny told supporters and the media upon landing, just minutes before he was detained. “Have you been waiting for me long?” he asked border guards.
His wife Yulia – who had flown with him from Germany – after police officers warned they would use physical force if he disobeyed their orders to come with them. Despite pleas, Mr Navalny’s lawyer was not allowed to accompany him.
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.
Russia’s prison service in a statement 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.
Separately, Russian prosecutors have launched a new criminal case against Mr Navalny on fraud charges related to transfers of money to various charities, including his Anti-Corruption Foundation.
The activist earlier argued that Mr Putin was fabricating new cases against him.