BRUSSELS/WASHINGTON (REUTERS, AFP, BLOOMBERG) - The detention of Alexei Navalny, an outspoken critic of Russia President Vladimir Putin on Sunday has drawn widespread criticism with European Council president Charles Michel condemning the detention of Navalny, urging the Russian authorities to free him.

Navalny was arrested as he arrived in Moscow after being treated in Germany for poisoning.

"The detainment of Alexei Navalny upon arrival in Moscow is unacceptable. I call on Russian authorities to immediately release him," Michel wrote on Twitter.

The EU's foreign policy chief Josep Borrell echoed the call for Navalny to be freed following his arrest. "Russian authorities must respect Alexei Navalny's rights and release him immediately. Politicisation of the judiciary is unacceptable," Borrell tweeted.

The foreign ministries of Germany, Italy and France have also called for the immediate release of Navalny.

"Russia is bound by its own constitution and by international obligations to the principle of the rule of law and to the protection of civil rights," Germany's foreign minister Heiko Maas said on Monday (Jan 18) in a statement. "These principles must, of course, be applied to Alexei Navalny as well. He should be released immediately."

"The arrest of Navalny on his arrival in Russia is a very serious matter, which concerns us. We ask for his immediate release. And we expect his rights to be respected", Italian foreign minister Luigi Di Maio wrote in a tweet.

"France notes the arrest of Mr Alexei Navalny in Russia with very strong concern," the French foreign ministry said in a statement on Sunday. "Along with its European partners, it is following his situation with the greatest vigilance and calls for his immediate release."

Italian Economy Minister Roberto Gualtieri on Twitter also asked for Navalny's immediate release.

The statement echoed calls for Navalny's release from US Secretary of State Michael Pompeo, who is due to depart with the rest of the Trump administration on Wednesday.

"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," Pompeo said in a statement on Sunday.

Similarly, President-elect Joe Biden's national security adviser, Jake Sullivan has also condemned Navalny's detention.

"Mr Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable," Mr Sullivan said on Twitter. "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."

Russia should immediately release Kremlin critic Alexei Navalny, who was detained on arrival in Moscow on Sunday after returning from Germany for the first time since his poisoning last summer, one of US President-elect Joe Biden's top aides said.

"Mr Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable," Jake Sullivan, Biden's incoming White House national security adviser, said on Twitter.

"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."

British Vaccine Deployment Minister Nadhim Zahawi said on Monday Britain is worried about the safety of Navalny after his detention.

​"The Foreign Secretary will say more about this, but we are very worried about the wellbeing and safety of Alexei Navalny. And of course, we have to make sure that the Russian government answers why a poison was used against Alexei Navalny," Zahawi told Sky.

Lithuania's Foreign Minister Gabrielius Landsbergis has called for EU sanctions against Moscow, while Amnesty International accused Russian authorities of waging "a relentless campaign" to silence Navalny.

Polish Prime Minister Mateusz Morawiecki asked for a "swift and unequivocal" response from the European Union toward Navalny's detention.

"The detention of Navalny is another attempt to intimidate the democratic opposition in Russia. A swift and unequivocal response at the EU level is essential. Respect for citizens' rights is the cornerstone of democracy," Morawiecki wrote on Twitter.

"I appeal to the authorities in Russia to release the detainee immediately," he wrote.