Alexei Navalny, Russia’s leading opposition figure, jailed for 5 years (PHOTOS)

Russia’s most prominent opposition figure, blogger and would-be mayor of Moscow Alexei Navalny, was sentenced to five years' imprisonment Thursday in a case critics say is payback for his political activities.

Navalny, who played a leading role in the anti-Kremlin protests that erupted in December 2011, was convicted by a provincial court of stealing more than $500,000 worth of timber from a state-run firm in Russia’s Kirov Region in 2009.

The anti-corruption blogger, who for several years helped expose evidence of alleged official graft, received a mandatory five-year sentence.

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While the verdict was widely expected, there was shock at the severity of the sentence, which could keep Navalny in jail for the next presidential election in 2018 — when, some had speculated, he could muster the momentum and support to challenge President Vladimir Putin. 

Unless Navalny successfully appeals the decision, his conviction will bar him from holding public office.

Just Wednesday, Moscow city officials accepted his candidacy for the capital’s mayoral elections slated for early September. That contest was to be the first official election in which Navalny took part.

However, his election chief, Leonid Volkov, told Agence-France Presse Thursday, "A decision has been made to boycott the elections."

"It would be strange if we participated in some sort of beating, turned the other cheek," Volkov said in televised remarks, regarding the mayoral race. He said Navalny would notify the Moscow Election Commission of his decision to boycott.

The United States ambassador to Russia, Michael McFaul, criticized the verdict in unusually strong terms:

The United Kingdom's foreign secretary, William Hague, expressed concern about “selective justice,” while European Union foreign policy chief Catherine Ashton said the ruling raised “serious questions as to the state of the rule of law in Russia.”

Navalny’s supporters called for a protest outside the Kremlin Thursday evening.

His conviction in some ways reflects the culmination of a Kremlin crackdown on dissent since Putin’s return for a third term as president last year.

The Russian parliament has in recent months passed a raft of legislation that critics say is aimed at stifling civil society. They include laws stiffening the punishment for public protesting, expanding the definition of treason, and banning “homosexual propaganda,” among others.