MOSCOW — Russia’s highest-profile opposition leader, Aleksei A. Navalny, was sentenced to 30 days in jail on Monday for a protest that happened seven months ago — a move that he maintained was intended to prevent him from organizing nationwide protests against plans to raise the retirement age in Russia.
The ruling, handed down by the Tverskoi District Court in Moscow, held that Mr. Navalny, whom the police detained on Saturday, broke the law by holding an unsanctioned protest in the capital in late January.
In a Facebook post on Sunday, Mr. Navalny accused the authorities of detaining him to disrupt the planning of nationwide protests scheduled for Sept. 9, and to prevent him from attending those demonstrations.
“Usually, they try to detain me on the eve of the protest, but this time it was a whole two weeks in advance,” he wrote as he awaited the latest in a long string of rulings sentencing him to short jail terms. It is his fifth such administrative arrest since the start of 2017.
President Vladimir V. Putin’s approval ratings have suffered since the government unveiled a proposal in June to gradually raise the retirement age to 65 from 60 for men and to 63 from 55 for women. The pension overhaul plan has divided Russia’s ruling party and energized its rivals.
The Kremlin has tried to distance Mr. Putin from that decision, and some modifications are expected. Still, Mr. Navalny has been an effective organizer of nationwide demonstrations, and the government typically fears any protest movement that could gain momentum.
With nationwide elections scheduled for Sept. 9, the Kremlin is taking no risks, analysts said. “The authorities do not want scandals” involving Mr. Navalny to mar the vote, Aleksei V. Makarkin, an analyst in Moscow, told the daily newspaper Nezavisimaya Gazeta.
In the protest last January, Muscovites took to the streets to voice their opposition to a fourth presidential term for Mr. Putin, who went on to handily win a March vote in which he faced no significant competition.
Mr. Navalny, who sought to run but was denied a place on the ballot because of an earlier conviction that he calls politically motivated, joined the protesters and was one of more than 15 people detained that day.
The protest was part of a daylong “voters’ strike” held across the country. In videos uploaded to his YouTube channel, Mr. Navalny urged Russians to rise up against Mr. Putin and his allies. “They’ve already spent 18 years in power, and now they want another six years,” Mr. Navalny said. “Only you and I can stop them.”
However, on Monday he disputed the premise of the charge against him, saying that because he had not submitted the formal request to hold the protest in Moscow, he could not be legally considered its organizer.
Mr. Navalny’s lawyer, Ivan Y. Zhdanov, said he had not been given a chance to review the 219 pages in the case files.