Russian police detained President Vladimir Putin’s critic Alexei Navalny at a Moscow airport on Sunday shortly after he landed on a flight from Berlin, AFP reported.

Navalny’s spokeswoman Kira Yarmysh confirmed he had been detained, saying on Twitter: “Alexei was taken away by police officers at the border. With no explanation given.”

A flight carrying Navalny back to Russia landed at a Moscow airport after it was diverted from a different airport where his supporters and media had gathered. The plane arrived at Sheremetyevo airport instead of Vnukovo as planned.

Navalny, 44, spent four months in Berlin, recovering from a nerve-agent poisoning that he blames on the Kremlin. Western experts concluded he was poisoned with Soviet-designed nerve toxin Novichok. The Kremlin denies any involvement in the attack.

Navalny has never held elected office. He came second in a 2013 vote for mayor of Moscow but was barred from standing against Putin in the 2018 presidential elections.

His allies are also frequently prevented from running for election.

In 2019, several Navalny allies were barred from running for the Moscow city council, sparking mass rallies in the capital that lasted several weeks.