Navalny was detained at passport control at Moscow’s Sheremetyevo airport after flying in Sunday evening from Berlin, where he was treated following the poisoning in August that he blames on the Kremlin.

Navalny’s arrest adds another layer of tension to relations between Moscow and the West that have long been strained and were worsened by his poisoning.

Also read: Kremlin critic Navalny detained after landing in Moscow

German Foreign Minister Heiko Maas noted that Navalny had returned of his own volition and said “it is completely incomprehensible that he was detained by Russian authorities immediately after his arrival.”

“Russia is bound by its own constitution and by international commitments to the principle of the rule of law and the protection of civil rights,” Maas added. “These principles must of course also be applied to Alexei Navalny. He should be released immediately.”

The politician’s allies said Monday he was being held at a police precinct outside Moscow and has been refused access to his lawyer. According to Navalny’s lawyers, in an unexpected turn of events, a court hearing into whether Navalny should remain in custody started on Monday at the precinct itself, and they were notified minutes before.

“It is impossible what is happening over here,” Navalny said in video from the improvised court room, posted on his page in the messaging app Telegram. “It is lawlessness of the highest degree.”

Calls for Navalny’s immediate release have also come from European Commission President Ursula von der Leyen, British Foreign Secretary Dominic Raab and top officials of other EU nations.

U.S. President-elect Joe Biden’s pick for national security adviser called on Russian authorities to free Navalny. “Mr. Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable,” Jake Sullivan tweeted.

Also read: Russian opposition politician Navalny poisoned, hospitalized

The outgoing U.S. secretary of state, Mike Pompeo, said the U.S. “strongly condemns” the decision to arrest Navalny, which he called “the latest in a series of attempts to silence Navalny and other opposition figures and independent voices who are critical of Russian authorities.”

Navalny’s detention was widely expected because Russia’s prisons service said he had violated parole terms from a suspended sentence on a 2014 embezzlement conviction.

The prisons service said it would seek to have Navalny serve his 3 1/2-year sentence behind bars.

Russian Foreign Minister Sergey Lavrov said Monday the stream of reactions to Navalny’s arrest by Western officials reflects an attempt “to divert attention from the crisis of the Western model of development.”

“Navalny’s case has received a foreign policy dimension artificially and without any foundation,” Lavrov said, arguing that his detention was a prerogative of Russian law enforcement agencies that explained their action. “It’s a matter of observing the law,” he added.

Navalny, 44, President Vladimir Putin’s most prominent and determined foe, brushed off concerns about arrest as he boarded his flight in Berlin on Sunday.

“It’s impossible. I’m an innocent man,” he said.

Navalny fell into a coma while aboard a domestic flight from Siberia to Moscow on Aug. 20. He was transferred from a hospital in Siberia to a Berlin hospital two days later.

Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.

Also read: Russian authorities raid opposition leader Navalny's offices

Russian authorities insisted that the doctors who treated Navalny in Siberia before he was airlifted to Germany found no traces of poison. Russia refused to open a full-fledged criminal inquiry, citing a lack of evidence that Navalny was poisoned, and officials have challenged Germany to provide proof of the poisoning.

Last month, Navalny released the recording of a phone call he said he made to a man he alleged was a member of a group of officers of the Federal Security Service, or FSB, who purportedly poisoned him in August and then tried to cover it up. The FSB dismissed the recording as fake.

Navalny has been a thorn in the Kremlin’s side for a decade, unusually durable in an opposition movement often demoralized by repression.