Russian opposition leader Alexei Navalny was arrested after arriving in Moscow on Sunday amid threats from the government.

In December, Russia's prison service gave the Kremlin critic a Dec. 29 deadline for returning to Moscow from Germany. If he returned after the deadline, Navalny would be jailed.

On Monday, Navalny tweeted that he would be detained for 30 days, until Feb. 15, according to an English translation.

According to the prison service, Navalny was in violation of the terms of a suspended prison sentence he is serving over a 2014 conviction and of evading the supervision of Russia's criminal inspection authority. Navalny, 44, is currently serving out a suspended 3.5-year prison term over a theft case he claims was politically motivated. His probation period expired on Dec. 30.

Navalny revealed he would be returning to Russia last Wednesday, and Moscow's prison service said they would arrest him upon his return, according to Reuters.

"This is the best moment in the last five months," he told reporters on the plane. "I feel great. Finally, I’m returning to my home town."

Navalny was accompanied by his wife, lawyer, and spokesperson on the flight. He was later seen hugging his wife before being detained by the authorities.

In an alleged effort by authorities to prevent supporters from greeting him, the flight was diverted to a separate Moscow airport at the last minute. Originally meant to land at Moscow's Vnukovo Airport, the flight landed at Moscow's Sheremetyevo airport.

Sunday's flight was Navalny's first trip back to Russia since he was poisoned last summer. After initially being held by doctors in Russia, he was released to Germany, where investigators determined he was poisoned with a Novichok-class nerve agent, a potent toxin produced in Russia since the time of the Soviet Union. Navalny recovered in Germany up until his return on Sunday.

While Navalny and the U.S. State Department accused Russian operatives of poisoning the vocal critic, Russian President Vladimir Putin said that if Russia wanted Navalny dead, forces would likely have “finished it.”

An investigation conducted by CNN and the online investigative outlet Bellingcat took thousands of phone records, flight manifests, and other documents to identify the team of toxin experts who were behind Navalny's poisoning. The team, made up of six to 10 agents, trailed Navalny for more than three years before the attack.