Russian opposition activist Alexei Navalny was arrested upon returning home with his wife Yulia after months of treatment in a German hospital.

Navalny was arrested at Sheremetyevo Airport, Moscow, on January 17, less than an hour after his return from Berlin.

The Navalny flight from Berlin was originally scheduled to land at Vnukovo airport, where hundreds of supporters were waiting to greet him.

The Russian prison regulator said the activist was arrested on charges of violating the probation rule and will be detained until the trial takes place later this month.

Olga Mikhailova, Navalny's lawyer, said the client was detained "without reason" and accused what was happening "were illegal.

The Russian arrest of Navalny also immediately drew criticism from the US and Western countries.

European Council President Charles Michel criticized the Navalny's arrest as "unacceptable", while the French Foreign Ministry said the move posed "great concern".

Russian Foreign Ministry spokeswoman Maria Zakharova countered the accusations, demanding that foreign leaders "respect international law" and "handle domestic affairs".

Upon boarding the plane back to Russia, Navalny said he believed he would not be arrested.

Navalny, 44, was seriously ill on a flight from Siberia to Moscow in August 2020 and was hospitalized in the Russian city of Omsk before being taken to Berlin, Germany, for treatment.

Experts in some Western countries concluded that Navalny was poisoned with the Soviet nerve agent Novichok, but Moscow repeatedly denied this allegation.