Earlier in the day, Navalny landed at Moscow's Sheremetyevo airport where he was quickly arrested by the Russian detention authority for multiple probation violations.
"The detainment of Alexey #Navalny upon arrival in Moscow is unacceptable. I call on Russian authorities to immediately release him," Michel tweeted.
On August 20, Navalny fell ill on a domestic flight in Russia. He was initially treated in the Siberian city of Omsk, after the plane made an emergency landing. Two days later, when doctors determined that he was fit for cross-border air transport, the Russian opposition vlogger was taken to the Berlin-based Charite hospital for further treatment.
In a statement, the German government said that doctors found evidence of a poisonous nerve agent from the Novichok group inside of Navalny's body. Russia disputed the allegations, noting that its doctors found no toxic substances in Navalny while he was being treated in Omsk.
Prior to Navalny's return, the Russian Federal Penitentiary Service warned that it would arrest him for probation violations, after he had previously been found guilty in court of committing two administrative offenses.