Officially, the demonstration is against corruption, but it is also an effort by Mr. Navalny to force the Kremlin to let him stand against President Vladimir V. Putin in the March 2018 presidential election, even if he has virtually zero chance of unseating the president. A felony conviction, which Mr. Navalny has called politically motivated, technically bars him from running.

On Monday, there were scattered reports of detentions in various cities.

In Vladivostok, home to Russia’s Pacific Fleet, at least 11 protesters were detained, according to OVD-Info, a Russia group that monitors arrests. An independent news website in the city, VL.RU, put the number “around 20.”

The crowd of hundreds, while constituting only a tiny portion of the city’s population of 600,000, signaled defiance against the authorities, who mobilized burly Cossacks in camouflage uniforms and czarist-era outfits as well as various police units to deter protesters from gathering in a central square opposite the terminus of the Trans-Siberian Railway.

After gathering in the square to chant against corruption and to wave copies of the Russian Constitution, which guarantees freedom of speech and assembly, the demonstrators paraded through narrow streets with Russian flags to an esplanade overlooking the Pacific Ocean.

“Russia without Putin,” they shouted, while one protest banner said “Power must be changeable,” a reference to tightly controlled elections that mostly consolidate the power of Mr. Putin and his allies.

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