Russian opposition leader Alexei Navalny was detained at Moscow airport after returning from Germany where he underwent treatment for a poisoning by nerve agent he blamed on the Kremlin.
The Moscow Directorate of Russia's Federal Penitentiary Service said he was detained for breaching the terms of his parole for a suspended prison sentence. It said he would be held in custody until a court could rule in his case.
Mr Navalny has spent the last five months in Germany recovering from the nerve agent attack on August 20 while flying from Tomsk to Moscow.
He was hospitalised and then flown to Germany for specialist treatment at the Charite university hospital in Berlin. The Organisation for the Prohibition of Chemical Weapons confirmed Mr Navalny had been poisoned with a nerve agent and said indications showed it to be a Novichok.
The Kremlin has repeatedly denied any part in the opposition leader’s poisoning.
The penitentiary service issued a warrant for his arrest last week, saying he had breached the terms of a suspended sentence handed in 2014 on charges of embezzlement.
It has also asked a Moscow court to return Mr Navalny’s to custody to serve his three and a half year term.
After boarding the flight for Moscow in Berlin on Sunday, Mr Navalny said of the prospect of arrest: “It’s impossible. I’m an innocent man.”
Mr Navalny's supporters went to Moscow’s Vnukovo Airport, where the plane was scheduled to land, but it ended up touching down at Sheremetyevo airport, about 40 kilometres away.
There was no immediate explanation for the flight diversion.
The OVD-Info group, which monitors political arrests, said at least 37 people were arrested at Vnukovo Airport, although their affiliations were not immediately clear.
Vnukovo banned journalists from entering the terminal last week, saying it was due to concerns about the coronavirus.
The airport also blocked access to the international arrivals area.
Police prisoner-detention vehicles stood outside the terminal on Sunday.
Independent newspaper Novaya Gazeta and opposition social media reported on Sunday that several of Mr Navalny's supporters in St Petersburg were removed from Moscow-bound trains or prevented from boarding flights late on Saturday and early on Sunday.
They included the co-ordinator of his staff for the region of Russia’s second-largest city.
When Mr Navalny was taken ill on August 20, he was put into a coma before being transferred from the hospital in Siberia to Berlin two days later.
Laboratories in Germany, France and Sweden, and tests by the Organisation for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.
Russian authorities insisted that the doctors who treated Navalny in Siberia before he was flown to Germany found no traces of poison and have challenged German officials to provide proof.
They refused to open a full-fledged criminal inquiry, saying there was a lack of evidence that Mr Navalny was poisoned.
Last month, the opposition figure released the recording of a phone call he said he made to a member of a group of officers from the Federal Security Service, who he claimed poisoned him in August and then tried to cover it up.
The FSB dismissed the recording as fake.
Joe Biden’s national security adviser, Jake Sullivan, is calling on Russia to immediately release Mr Navalny, saying "the perpetrators of the outrageous attack on his life must be held accountable".
"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," Mr Sullivan said.
The high representative of the EU for foreign affairs, Josep Borrell, also called for Mr Navalny’s immediate release and warned against the “politicisation of the judiciary” in Russia.
The President of the European Council Charles Michel also called for the immediate release of Mr Navalny describing his detention as “unacceptable”.
The governments of Latvia, Lithuania and Estonia warned Russia to release Mr Navalny or face the prospect of new EU sanctions.