Kremlin critic Sergei Udaltsov who went on hunger strike after being jailed for 30 days for burning portraits of Russian leaders was hospitalised again on Sunday, his wife said.
Earlier this month, Udaltsov was convicted of torching the portraits during a protest against deeply unpopular plans to raise the Russian retirement age.
He was taken to hospital due to a "worsening of his condition on the 13th day of his hunger strike", his wife Anastasia Udaltsova said, quoted by the official TASS news agency.
The far-left activist was previously hospitalised on August 19, five days into his hunger strike, when he was taken to a Moscow hospital suffering from dehydration after doctors were called to his detention centre to take blood tests.
"I just saw him in hospital and talked to the doctors who are treating him. He is starting to suffer from kidney failure and his body is very dehydrated. I hope that he will remain for a few days on the drip," she said.
Last year he finished a four-and-a-half-year prison sentence for organising anti-Kremlin protests.
Udaltsov 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.
He went on hunger strike several times during those years spent in a penal colony in central Russia's Tambov province.