- Police arrested Navalny, 44, at Moscow's Sheremetyevo airport on Sunday shortly after his flight from Berlin, Germany landed in the country's capital city.
- Navalny had been recuperating in Germany after narrowly surviving what has since been independently confirmed as poisoning by a Novichok nerve agent on August. 20.
- European Council President Charles Michel on Sunday described Navalny's arrest as "unacceptable" and called on Russian authorities to "immediately release him."
LONDON — The U.S. and several European governments have expressed deep concern following the arrest of Kremlin critic Alexei Navalny, urgently calling for his immediate release from Russian detention.
Police arrested Navalny, 44, at Moscow's Sheremetyevo airport on Sunday shortly after his flight from Berlin, Germany landed in the country's capital city.
The activist, who is widely regarded as the most prominent and determined critic of Russian President Vladimir Putin, was returning home for the first time since he was poisoned last summer.
Navalny had been recuperating in Germany after narrowly surviving what has since been independently confirmed as poisoning by a Novichok nerve agent on August. 20.
The opposition politician believes Putin ordered the poisoning to go ahead, reportedly saying in October last year that he does not see any other explanation.
Putin's government denies poisoning Navalny, though investigative reporters have since published evidence to support Navalny's claims.
U.S. Secretary of State Mike Pompeo said via Twitter on Monday that he was "deeply troubled" by the development and called for his immediate and unconditional release from detention.
"Confident political leaders do not fear competing voices, nor commit violence against or wrongfully detain political opponents," Pompeo said.
Separately, President-elect Joe Biden's national security advisor Jake Sullivan pushed for Navalny's immediate release. Sullivan said: "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," he added.
In response to calls from Western nations calling for Navalny's immediate release, Moscow said his case had received "artificial" resonance in the West.
Speaking onboard the plane in Berlin ahead of takeoff, Navalny had said he did not expect to be arrested when he arrived in Russia.
His flight had been due to land at Vnukovo airport where supporters and media had been waiting despite bitterly cold weather, but his route was reportedly diverted to Sheremetyevo airport due to "technical reasons."
"I feel great. Finally, I'm returning to my home town," Navalny said aboard the flight back to Moscow, according to a Reuters report. Navalny was accompanied on the flight by his wife, Yulia, as well as his spokesman and lawyer.
On arriving in Moscow on Sunday evening, Navalny was last seen saying goodbye to his wife at passport control before being led away by Russian authorities.
In a video posted on Twitter by Kira Yarmysh, Navalny's spokesperson, he was shown complaining about the absurdity of makeshift legal proceedings at Khimki police station, near Moscow's Sheremetyevo airport. "It doesn't get more lawless than this," he said, according to an NBC translation.
Allies of the Kremlin critic fear he may soon be sentenced to time in prison on trumped-up charges. Navalny's detention had been ordered by Moscow's prison service in relation to alleged violations of a suspended prison sentence.
"Aleksei Navalny's arrest is further evidence that Russian authorities are seeking to silence him," Natalia Zviagina, Amnesty International's Moscow Office Director, said in a statement on Sunday.
"His detention only highlights the need to investigate his allegations that he was poisoned by state agents acting on orders from the highest levels," Zviagina said.
The Russian Embassy in London did not immediately respond to CNBC's request for comment.
In Europe, several world leaders issued statements sharply critical of Navalny's arrest, but most stopped short of calling for punitive action.
European Council President Charles Michel on Sunday described Navalny's arrest as "unacceptable" and called on Russian authorities to "immediately release him."
The foreign ministries of the U.K., Germany, France and Italy all separately issued statements to rebuke Navalny's arrest and demand his immediate release.
Finnish Prime Minister Sanna Marin said via Twitter on Monday that Navalny must be released "without delay," adding that others arrested on his arrival should also be freed. "Russia should investigate Navalny's poisoning, protect rights of opposition which belongs to any democracy," Marin said.
A joint statement by the Baltic states of Estonia, Latvia and Lithuania, three former Soviet republics, issued a call for the EU to consider the "imposition of restrictive measures in response to this blatant act" if Navalny is not released from detention.
They described Navalny's arrest as "completely unacceptable."