Navalny, 44, a prominent opponent of Russian President Vladimir Putin, started refusing food on March 31 in protest at what he said was the refusal of prison authorities to treat him properly for acute back and leg pain.
"His condition is indeed critical," said Alexandra Zakharova, a representative of the Doctors Alliance trade union - a group that Russian authorities regard as opposition activists.
Navalny was arrested on his return to Russia in January, after spending months in Germany recovering from a poisoning last summer that he blames on the Kremlin.
Farm leaders said Saturday’s hunger strike, to coincide with the death anniversary of Indian independence leader Mahatma Gandhi, would show Indians that the protesters were overwhelmingly peaceful.