MOSCOW (AP) — A Moscow court sentenced Russian opposition leader Alexei Navalny on Monday to a month in jail for an unsanctioned protest, a move that puts President Vladimir Putin's most prominent political rival behind bars and not in the streets for the next nationwide anti-government protest.

Navalny's arrest Saturday outside his home came as a surprise, since police were detaining him over a protest rally held in January. Navalny has been jailed multiple times for organizing demonstrations, but that typically happens soon after the event.

Navalny has called for nationwide rallies on Sept. 9 to protest the Russian government's plans to raise the retirement age for both men and women. The issue has created widespread outrage, uniting Russians with widely varying political views against the proposal.

After the Tverskoy district court ruled Monday to put Navalny in custody for 30 days, he urged his supporters to still take to the streets for the pension protest. In a tweet posted shortly after the ruling, Navalny said the Kremlin "shouldn't think that my arrest changes anything."

Navalny rose to prominence thanks to his investigations into official corruption. On Friday he published another investigation, alleging that at least $29 million had been stolen in procurement contracts for Russia's National Guard, which is headed by Putin's former bodyguard, Viktor Zolotov.