Police detained prominent Kremlin critic Alexei Navalny on arrival at a Moscow airport today after he flew home to Russia from Germany for the first time since he was poisoned last summer.
The move, which could see Mr Navalny jailed for three and a half 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, Mr Navalny was poisoned last summer by what German military tests showed was a Novichok nerve agent, a version of events the Kremlin rejects.
Mr 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 Mr 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 his plane, saying he was not afraid and did not believe he would be arrested.
In the event, he was swiftly detained when he showed his passport to border guards before formally entering Russia, witnesses said. His wife, Yulia, his spokeswoman, and his lawyer were allowed to enter Russia.
FSIN said in a statement that Mr 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 that will rule whether to convert his suspended sentence into a jail term.
Mr Navalny, one of Putin's most prominent domestic critics, faces potential trouble in three other criminal cases too, all of which he says are politically motivated.
Mr Navalny said Mr Putin was behind his poisoning. The Kremlin has denies involvement, saying it has seen no evidence that he was poisoned, and that he was free to return to Russia.
Mr Navalny said the Kremlin is afraid of him.
The Kremlin, which only refers to him as the "Berlin patient", dismisses that.
Mr Putin's allies point to opinion polls that show the Russian leader is far more popular than Mr Navalny, whom they call a blogger rather than a politician.
For around a decade, Mr Navalny has been the symbol of Russia's protest movement, after rising to prominence as an anti-corruption blogger and leading anti-government street rallies.
His team publishes YouTube investigations into the wealth of Russia's political elites, some of which garner millions of views, making them a target of lawsuits, police raids and jail stints.