Navalny, 44, was met by officers at passport control as he landed in Moscow on a plane from Berlin Sunday, according to a live video feed on his YouTube channel. The Federal Penitentiary Service said he had been detained for violating the terms of a suspended sentence, state-run Tass reported.

Navalny, whose anti-corruption exposes and success in galvanizing anti-government votes have increasingly needled the authorities, had been recovering in Germany from an August nerve-agent attack he and Western governments blamed on Putin.

“This is my home,” he told reporters who’d traveled with him Sunday shortly before he was detained. “I’m not scared of anything.” Navalny boarded the flight knowing that he could face a lengthy prison term if he returned

He kissed his wife, Yuliya, goodbye before walking off with police. Authorities said he would be held pending a court decision on his sentence later this month.

Dozens of his supporters were detained by police at Moscow’s Vnukovo Airport, where he had appealed to them to meet him. The flight was diverted to another airport shortly before arrival.

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The move to imprison the most prominent opponent of the Russian president marks the biggest crackdown by Putin in recent years. Coming days before U.S. President-elect Biden takes office, it could trigger an immediate clash with the new Democratic administration.

Navalny’s return comes as political tensions are rising ahead of parliamentary elections this autumn and support for the Kremlin falters amid the coronavirus downturn. Putin, 68, whose two-decade rule makes him the longest-serving leader since Soviet dictator Josef Stalin, last year overturned term limits, allowing him to stay in power until 2036. Speculation that he may step down far sooner is building.

“There were only two choices for Navalny — to stay in Germany or come home. If he remained an émigré, in Russia people would quickly lose interest in him,” said Alexei Makarkin, deputy director of the Center for Political Technologies in Moscow. “His calculation is he’ll become a symbol of resistance behind bars and a big risk for Putin.”

Authorities issued an arrest warrant for Navalny after accusing him of violating the conditions of a 3 1/2 year suspended fraud sentence by not appearing in person for regular summons. It asked a Moscow court on Jan. 11 to replace it with a prison term. The opposition politician also potentially faces separate charges of embezzlement punishable by up to 10 years in prison.

While for years Navalny was repeatedly jailed for weeks at a time and faced assaults on the street — at one point nearly losing his eye — the poisoning attack marked the most serious attempt to kill him. Russia denied any involvement and said it found no proof the opposition politician was poisoned, accusing him of fabricating it as part of working for the U.S. Central Intelligence Agency.

With Navalny ignoring the threat to arrest him, the Russian authorities’ response looks set to further sour ties with a once-close partner, Germany, and other European states led by France.

German Chancellor Angela Merkel, whose country offered treatment to Navalny after his near-fatal poisoning during a campaign trip to Siberia, visited him in hospital and provided personal security guarantees. The European Union in October sanctioned six Russian officials over the use of the banned Novichok chemical weapon in the attack.