MOSCOW—Russians nationwide protested President Vladimir Putin’s order to mobilize hundreds of thousands of people for the war in Ukraine, as Russia’s beleaguered activists hold out despite a clampdown on dissent that has sent hundreds to jail and driven tens of thousands to flee the country. In more than two dozen cities across Russia on Saturday, Russians demonstrated against the Kremlin’s decision this week to mobilize more than 300,000 to address manpower shortages in Ukraine, where the Russians recently suffered a stinging defeat and lost swaths of territory.

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