MOSCOW (AP) — A Russian court on Friday sentenced a close ally of opposition leader Alexei Navalny to a month in jail for organizing an unauthorized rally.

The district court in the city of Nizhny Novgorod on Friday found Leonid Volkov guilty of breaking the law on public gatherings. Volkov is the chief of Navalny's presidential campaign, and his jailing is a heavy blow for Navalny who wants to run for president against Vladimir Putin in the March election.

Navalny and his allies have repeatedly served jail terms connected to rallies critical of the Russian government. Navalny himself spent 20 days in jail in October for organizing another rally.

Demonstrations called for by Navalny this year were some of the largest in years, rattling the Kremlin.

Navalny tweeted on Friday that Volkov's jailing aims to jeopardize his campaign.

"Putin and the entire Kremlin are scared of our campaign and the support that we have," Navalny said. "It's important for them to paralyze our work."

The 41-year old anti-corruption crusader, who has been a persistent thorn in the Kremlin's side, has mounted a grassroots campaign unlike anything Russian politics has seen since the early 1990s. His campaign has resonated not only with opposition-leaning voters in big cities, but also with residents of smaller towns which have traditionally been Putin's turf.

The upper chamber of the Russian parliament is expected later this month to officially announce the start of the presidential campaign and call the date of the vote. Navalny has been convicted on two separate sets of charges that were largely viewed as politically motivated. One of the convictions bars him from running for president, but Navalny hopes that campaigning could get the authorities to overturn the ban.