MOSCOW—Thousands of people across Russia took to the streets in support of jailed opposition leader Alexei Navalny, in what has shaped up to be a key test for his movement and a fresh challenge for the Kremlin.
Police in riot gear detained hundreds of protesters during the unsanctioned rallies in dozens of Russian cities on Saturday, including the capital Moscow, according to independent monitors. Mr. Navalny, who was detained on Sunday upon his return to Russia from Germany following a near-fatal poisoning with a nerve agent, had called for protests and organizers said they expected rallies in more than 60 cities across Russia’s 11 time zones.
For Mr. Navalny, who has shown an ability to mobilize his large following into demonstrations, Saturday’s protests are a new test of his clout after being out of the country for five months, while supporters are facing freezing temperatures and increasing pressure from authorities. The strength and…