Russian activist Sergei Udaltsov was transferred to a Moscow hospital on Sunday after suffering from dehydration — five days into his hunger strike.

Udaltsov's hospitalization was reported by the TASS news agency, citing a human rights monitor who had visited him in jail.

"In our presence, the doctor ... made a decision about the need for urgent hospitalization," said Ivan Melnikov, of Moscow's public monitoring commission.

TASS reported the activist had also complained of kidney pain.

Udaltsov's supporters later confirmed his hospitalization on his social media channels.

Read more:Russian opposition activist Nadezhda Mityushkina wins Nemtsov Prize

Thorn in Putin's side

The 44-year-old is the unofficial leader of the Vanguard of Red Youth (AKM), a radical socialist group, and has been arrested several times for his opposition to Russian President Vladimir Putin's rule.

Most recently, he took part in demonstrations, in July, against deeply unpopular plans to raise the Russian retirement age.

Although the rallies had been given approval and were attended by some 6,000 people, Udaltsov was arrested after allegedly burning images of members of the Russian government. He was sentenced to 30 days behind bars.

Udaltsov's supporters said on his Facebook page they planned a rally in central Moscow on Monday to protest what they say is a fabricated case against Udaltsov.

The activist has repeatedly been arrested for taking part in anti-Putin protests. Last year, he finished a four-and-a-half-year prison sentence for organizing illegal rallies. He and co-defendant Leonid Razvozzhayev were convicted in July 2014 of fomenting mass riots across Russia ahead of Vladimir Putin's inauguration to a third term as president in 2012.

Read more: Vladimir Putin calls for unity and 'deep-seated change' after election victory

Multiple hunger strikes

Udaltsov went on hunger strike several times during those years at a penal colony in central Russia's Tambov province.

Meanwhile, concerns are mounting for another hunger-striking prisoner in a Russian jail. Ukrainian filmmaker Oleg Sentsov is reported to be seriously ill, after refusing food for more than 95 days.

Sentsov was convicted in 2015 of carrying out arson attacks on pro-Moscow party offices in the Russian-annexed Crimea region of Ukraine a year earlier. The US and EU have condemned those charges as politically motivated.

Sentsov started his protest on May 14 to demand the release of all Ukrainian political prisoners in Russia. But since then, his cousin Natalia Kaplan says Sentsov's health has declined dramatically.

Last week, the Kremlin denied a request by Sentsov's mother for a pardon, saying it would have to come from the prisoner himself.

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mm/jlw (AFP, TASS)

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