Russian opposition leader Alexei Navalny, right, attends an administrative hearing in a Moscow court. (Maxim Shipenkov/Epa-Efe/Rex/Shutterstock/Shipenk/Epa-Efe/Rex/Shutterstock)

Russian opposition leader Alexei Navalny, who has declared his intention to challenge incumbent Vladimir Putin in the 2018 presidential election, was sentenced Monday to 20 days in jail amid a crackdown by the Kremlin on dissent ahead of the vote.

 A Moscow court approved an administrative arrest for Navalny after he called on supporters to join an unsanctioned rally in the regional capital of Nizhny Nov­gorod as part of a whistle-stop tour across Russia to whip up opposition support.

Putin has not officially declared that he will run in the March presidential election. But media and political insiders here are speculating about when, not if, Putin will declare his candidacy. His reelection to a fourth presidential term is widely expected, an outcome that would prolong his role as the country’s most powerful figure to 24 years.

Speaking after the ruling, Navalny called the sentence a “birthday present for Putin.” The opposition leader’s supporters plan to rally in St. Petersburg on Saturday, which marks Putin’s 65th birthday, to protest the court decision.

Russian election officials have previously said that Navalny is not eligible to run for president in 2018 because he is serving a suspended sentence for fraud, a felony conviction that precludes his holding higher office. Navalny, who calls that ruling politically motivated, has said that he will run regardless and is holding rallies in cities across Russia’s 11 time zones, from Arkhangelsk in the north to Vladivostok in the Far East. 

Navalny’s supporters have complained of government interference, and a top campaign aide was recently attacked with a steel pipe in Moscow.

The opposition leader rose to prominence first as a corruption whistleblower, revealing self-
dealing in state companies, before becoming a protest leader during the demonstrations that erupted in the wake of Putin’s disputed election in 2012.

Putin served as president of Russia for two terms from 2000 until 2008. The former intelligence chief then served as prime minister, sidestepping a constitutional limit that restricts the president to two consecutive terms, before returning to the presidency in 2012. New legislation has extended the presidential term to six years.

Last month, Putin became the longest-serving leader in the Kremlin since Joseph Stalin, quietly surpassing Leonid Brezhnev’s 6,601 days in office.