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.