LONDON - Western leaders have demanded the release of Russian opposition leader Alexei Navalny, who was arrested Sunday on arrival in Moscow after receiving medical treatment in Germany.

The 44-year-old Putin critic and leader of the Russia of the Future party was poisoned by a nerve agent in Siberia last August. The Kremlin denies involvement in that attack.

“The Russian authorities have arrested the victim of an attempted assassination with a chemical weapon, not the perpetrator,” German government spokesperson Steffen Seibert told reporters Monday. “The German government strongly calls on the Russian government to first of all release Mr. Navalny immediately and secondly to fully investigate the circumstances of the chemical weapons attack on Russian soil,” he added.

Other European leaders echoed that demand, while the United Nations’ human rights office said it was “deeply troubled” by Navalny’s arrest.

U.S. Secretary of State Mike Pompeo used the same words on Twitter: “Deeply troubled by Russia's decision to arrest Aleksey Navalny. Confident political leaders do not fear competing voices, nor see the need to commit violence against or wrongfully detain, political opponents.”

President-elect Joe Biden’s incoming national security adviser Jake Sullivan tweeted: "Mr. Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable. The Kremlin’s attacks on Mr. Navalny are not just a violation of human rights, but an affront to the Russian people who want their voices heard.”

Navalny was arrested as he entered passport control in Moscow’s Sheremetyevo airport Sunday evening. Dozens of his supporters, who had gathered at a different airport before Navalny’s plane was diverted, were also taken into custody. There were further arrests Monday in St. Petersburg as his supporters staged street protests.

Police officers detain a man at Moscow's Vnukovo airport where Russian opposition leader Alexei Navalny is expected to arrive, outside Moscow, Russia, Jan. 17, 2021.

In a rapidly convened, highly unusual judicial hearing Monday inside Khimki police station on the outskirts of the capital, a judge ruled that Navalny should be held in pretrial detention for 30 days for breaching the conditions of a suspended jail sentence.

Prosecutors want the 3½-year suspended sentence for embezzlement converted into a full jail term. Navalny says the charges are politically motivated. In a mobile phone video recorded from inside the police station Monday, he said the hearing was a “mockery of justice.”

“I do not understand why the hearing of the Khimki court is taking place at a police station. Why no one was informed or notified? I have seen a lot of times the mockery of justice, but it looks like the old man in his bunker (Putin) is so scared of everything, that the criminal procedures' code has been blatantly torn up and thrown into the garbage. The thing which is happening here is simply impossible. This is the highest degree of lawlessness,” Navalny said.

He called on his supporters to take to the streets.

“Don't be silent, resist, take to the streets. Nobody would protect us but ourselves. But we are so many that if we want to achieve something, we will achieve it,” he said.

Navalny tried to run for president in 2018 but was barred following his conviction for embezzlement, which he says was fabricated. His return to Russia presents a challenge to President Vladimir Putin, says Russia analyst Ben Noble of University College London.

“With the State Duma elections coming up in 2021, not only is Navalny a visible demonstration of opposition to the Kremlin, his anti-corruption foundation and its production of very slick YouTube videos with evidence of corruption of state officials and other members of the economic elite in Russia are an embarrassment for the Kremlin. It’s certainly a danger for the Kremlin that Navalny turns into a martyr or his status as a martyr is consolidated,” Noble told VOA.

Western nations have so far stopped short of punitive action. The EU last month joined allies in adopting a so-called Magnitsky Act, which allows for sanctions on individuals accused of human rights abuses. Financier Bill Browder, who was instrumental in pushing the legislation following the death of his lawyer Sergei Magnitsky in a Moscow jail, told VOA it’s time the laws were put into action.

“If you include all the countries of the EU, the U.S., Canada, the U.K., there are 31 countries. And so that will have a very significant effect on Russia, on the people who are sanctioned. And as I understand it, Navalny and his team, before they got on the plane, have constructed some type of list of who they think should be on this sanctions list.”

Putin sees that outcome as the lesser risk, Noble said.

“The logic I imagine in the Kremlin will be that they can deal with that short-term turbulence; but getting him – this is an expectation – getting him in prison would be a way of depriving him of that media exposure,” Noble said. 

In a press conference Monday, Russian Foreign Minister Sergei Lavrov dismissed criticism of Navalny’s arrest.

“Western politicians view this as an opportunity to draw attention away from the deepest crisis the liberal development model has found itself in,” Lavrov said.

Analysts say Russia’s actions will exacerbate tensions with the West, as both sides prepare for a new U.S. administration to take power this week under President-elect Joe Biden.