Sergei Udaltsov, a prominent Russian opposition activist imprisoned over a May 2012 protest against Vladimir Putin and his government, has been released after serving a 4 1/2-year term.

Udaltsov was freed from a penitentiary in the Tambov region southeast of Moscow on August 8, his wife, Anastasia Udaltsova, wrote on Facebook.

Udaltsov, leader of the opposition group Levy Front (Left Front), was placed under house arrest months after a protest on Moscow's Bolotnaya Square on May 6, 2012 -- a day before Putin's inauguration to a third presidential term.

In July 2014 he was convicted of organizing "mass disorder" in connection with the protest, which erupted into violence that demonstrators and police blame on each other.

More than 400 people were detained in connection with the protest, part of a series of demonstrations that drew Russians angry over evidence of widespread fraud in December 2011 parliamentary elections and dismayed by Putin's return to the presidency after four years as prime minister.

More than 30 people were prosecuted in what came to be known as the Bolotnaya Case, and more than 20 were sentenced to prison or served time in pretrial custody.

Kremlin opponents and rights activists say that the prosecutions are part of a Kremlin campaign to stifle dissent and suppress protests during Putin's current term. They contend that protesters were provoked and that the state's claims of violence were strongly exaggerated.