Anticorruption blogger Aleksei Navalny traveled from Moscow to Astrakhan to join the protest.
Oleg Shein from the leftist A Just Russia party lost to a candidate from the ruling United Russia party in the March 4 local elections, according to official results.
Shein claims the election was rigged, and began a hunger strike.
Around 20 people are now fasting in support of his cause.
Shein says he will continue his hunger strike until authorities call a recount or a rerun of the election.
Meanwhile in Moscow, Left Front leader Sergei Udaltsov was detained at a picket in support of the Astrakhan hunger strikers.