A Russian opposition leader, politician, and anti-corruption activist who became internationally recognized for his outspoken criticism of Russian President Vladimir Putin and seemingly subsequent poisoning has been ordered to remain in custody for 30 days during an unexpected hearing in Russia.

According to CNN, on Monday (Jan. 18), Kremlin critic Alexei Anatolievich Navalny was ordered to remain in custody for nearly a month upon returning to the country, five months after he'd been poisoned with the nerve agent Novichok.

The opposition leader flew back to Moscow from Germany on Sunday, and was immediately detained by masked officers, CNN reports. The 44-year-old father of two was detained overnight at a police station in the city of Khimki, on the outskirts of Moscow.

Navalny was added to the country's federal wanted list in December for violating terms of probation related to a 2014 conviction for fraud, which he dismisses as politically motivated.

Russia's Federal Penitentiary Service (FSIN) has requested that a court replace his suspended sentence with a prison term, CNN reports. The news outlet adds that if the request is granted, Navalny will likely be jailed for 3.5 years.

On Monday morning, Navalny faced a surprise hearing in a temporary court that had been set up in the police station. Navalny's supporters labeled the hearing a "circus."

His lawyers said they were handed a notice about the proceedings mere minutes before it was scheduled to begin, and didn't have a chance to review any documents or even speak with Navalny.

Navalny was escorted out of a cell moments later under the impression he would finally be able to meet his defense team, but found himself in the court hearing, CNN reports.

He took to Instagram to post a video stating: "Some people are filming me, some people are sitting here, this is all called an open hearing of the Khimki court. They are reviewing the matter of me remaining in custody. Why is the hearing taking place inside of a police station? Why was no notice given? I've seen many instances of mockery of justice but the grandpa in the bunker is so scared they just tore apart the codex of procedures."

His spokesperson Kira Yarmysh remarked that the only people who appeared to have known in advance about the hearing were media from a pro-Kremlin tabloid.

Navalny has been a perennial thorn in Russian President Vladimir Putin's side, raising concerns for his safety in the country.

A joint investigation by CNN and the group Bellingcat accused the Russian Security Service (FSB) in Navalny's August poisoning, detailing the process involving an elite unit at the agency following Navalny's team during a trip to Siberia, when Navalny fell ill from exposure to military-grade Novichok.

The investigation also found that this unit, which included chemical weapons experts, had followed Navalny on more than 30 trips to and from Moscow since 2017, CNN reveals. Russia denies involvement in Navalny's poisoning. Putin himself said in December that if Russian security services had wanted to kill Navalny, they "would have finished" the job.

Despite this, several Western officials and Navalny himself continue to openly blame the Kremlin.

Governments around the world have, likewise, criticized Russia over Navalny's arrest. US Secretary of State Mike Pompeo, UK Foreign Minister Dominic Raab, and German Foreign Minister Heiko Maas have all called for his immediate release in the past day, CNN notes.

Pompeo said the "United States strongly condemns Russia's decision to arrest Aleksey" in a statement Sunday. "We note with grave concern that his detention is the latest in a series of attempts to silence Navalny and other opposition figures and independent voices who are critical of Russian authorities."