Putin's critic Alexei Navalny arrested moments after release from jail

Alexei Navalny (right) is expected to appear in a Moscow court later on Monday

Russia's opposition leader Alexei Navalny has been detained again - moments after he had finished serving a 30-day sentence, his colleagues say.

He is accused of violating a protest law and was sentenced to another 20 days in jail.

The sentence Mr Navalny had just served was over planning an unauthorised anti-government demonstration in January.

The 42-year-old politician has long been the most prominent face of Russian opposition to President Vladimir Putin.

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Mr Navalny and his supporters believe the pressure he is coming under is politically motivated, the BBC's Steven Rosenberg in Moscow reports.

In recent weeks, the opposition politician has called for nationwide protests against government plans to raise the retirement age in Russia - a deeply unpopular reform that is fuelling resentment with the authorities and which has dented Mr Putin's popularity, our correspondent says.

Earlier this month, the head of Russia's National Guard Viktor Zolotov had threatened to make "a juicy beefsteak" out of Alexei Navalny, after the political activist had published allegations of large-scale corruption within the National Guard.

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Mr Navalny has called Mr Putin's party a place of "crooks and thieves", accused the president's system of "sucking the blood out of Russia" and vowed to destroy the "feudal state" being built.

The Kremlin denies the allegations.

Mr Navalny's candidacy in the 2018 presidential election was banned by the authorities over his conviction by a Russian court of embezzlement, which barred him from running for office.