Leftist leader Udaltsov was sentenced in July 2014 to four-and-a-half years on accusations of fomenting mass riots ahead of President Vladimir Putin's inauguration to a third term in 2012.
Human rights organisations say his prosecution was part of a politically-motivated crackdown to silence opposition to Putin and that the harsh punishment was designed to dissuade future protests against the Kremlin strongman.
Udaltsov's wife Anastasia told AFP that the her husband had started a hunger strike Saturday to protest being "illegally" transferred to solitary confinement in his penal colony in central Russia.
Udaltsov has frequently gone on hunger strike and last year launched a similar protest shortly after he was handed the guilty verdict at the end of his lengthy trial.