Prominent Kremlin critic Alexei Navalny has been detained by police on arrival at a Moscow airport on Sunday (local time) after he returned home from Germany for the first time since he was poisoned last summer.
The move, which could see Navalny jailed for 3.5 years for allegedly flouting the terms of a suspended prison sentence, is likely to spark a wave of Western criticism of President Vladimir Putin.
In a case that drew wide international attention, Navalny was poisoned last summer by what German military tests showed was a Novichok nerve agent, a version of events the Kremlin rejects.
Navalny's plane from Berlin was diverted to another Moscow airport at the last minute in an apparent effort by authorities to thwart journalists and supporters greeting him.
After Navalny said last week he planned to return home, the Moscow prison service (FSIN) said it would do everything to arrest him once he returned, accusing him of flouting the terms of a suspended prison sentence for embezzlement, a 2014 case he says was trumped up.
But the 44-year-old opposition politician laughed and joked with journalists on the plane, saying he was not afraid and did not believe he would be arrested.
Navalny was swiftly detained when he showed his passport to border guards before formally entering Russia, Reuters witnesses said. His wife, Yulia, his spokeswoman, and his lawyer were allowed to enter Russia.
FSIN said in a statement Navalny had been detained due to the alleged violations of his suspended prison sentence, and would be held in custody until a court hearing later this month. The hearing will rule whether to convert his suspended sentence into a real 3.5 year jail term.
Navalny, one of Putin's most prominent domestic critics, also faces potential trouble in three other criminal cases, all of which he says are politically motivated.
Navalny says Putin was behind his poisoning. The Kremlin denies involvement, says it has seen no evidence that he was poisoned, and that he was free to return to Russia.
Navalny says the Kremlin is afraid of him - a claim the Kremlin, which only refers to him as the "Berlin patient", laughs off. Putin allies point to opinion polls that show the Russian leader is far more popular than Navalny, whom they call a blogger rather than a politician.
European Union members Lithuania, Latvia and Estonia called for the "imposition of restrictive measures" against Russia following Navalny's arrest.
Navalny took a flight operated by Russian airline Pobeda, owned by state-controlled Aeroflot.
His supporters gathered at Moscow's Vnukovo airport despite a forecast of bitterly cold minus 22 Celsius weather and over 4,500 new coronavirus cases a day in the Russian capital.
The authorities' decision to switch airports to Sheremetyevo airport, ostensibly for technical reasons, thwarted them.
OVD-Info, a monitoring group, said police had detained 53 people in Moscow and five in St Petersburg.
The Moscow prosecutor's office, which had officially warned 15 pro-Navalny organisers, had said meeting him en masse was illegal because it was not sanctioned by the authorities.