Alexei Navalny has been detained after flying back to Russia five months after he was nearly killed by a nerve agent attack last year.
The 44-year-old Kremlin critic was seen being led away by police at passport control.
Big crowds earlier gathered at Moscow’s Vnukovo airport to greet his flight from Berlin but the plane was rerouted.
Alexei Navalny says the authorities were behind the attempt on his life, an allegation backed up by investigative journalists but denied by the Kremlin.
European Council President Charles Michel described Alexei Navalny’s detainment as “unacceptable”.
He tweeted: “I call on Russian authorities to immediately release him.”
Alexei Navalny told his supporters and the media at Moscow’s Sheremetyevo airport just minutes before his detention: “I know that I’m right. I fear nothing.”
His lawyer was not allowed to accompany Alexei Navalny. He kissed his wife Yulia – who had flown with him from Germany – after police officers warned they would use physical force if he disobeyed their orders.
On January 17, extra riot police were deployed and metal barriers erected inside Vnukovo airport, where the plane had been originally scheduled to land.
Russian media reported that a number of activists – including key Navalny ally Lyubov Sobol – were detained.
Alexei Navalny – who received medical treatment in Germany after his poisoning – urged supporters to meet him off the flight. Thousands of people said they would go or expressed an interest, despite forecasts of extreme cold and the ongoing Covid-19 pandemic.
The activist 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 Alexei Navalny’s claims that President Vladimir Putin himself ordered it.
In a statement late on January 17, Russia’s penitentiary service said Alexei Navalny “had been wanted since December 29, 2020, for repeated violations of the probation period”.
The penitentiary service added that he would remain in custody until a court decision.
The authorities accuse Alexei Navalny 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 the activist on fraud charges related to transfers of money to various NGOs, including his Anti-Corruption Foundation.
Alexei Navalny has asserted that President Vladimir Putin is doing all he can to stop his opponent from coming back by fabricating new cases against him.