MOSCOW—Russian police arrested more than 3,000 people Saturday in nationwide protests demanding the release of opposition leader Alexei Navalny, the Kremlin’s most prominent foe, according to a group that counts political detentions. The protests in scores of cities in temperatures as low as minus-50 C (minus-58 F) highlighted how Navalny has built influence far beyond the political and cultural centers of Moscow and St. Petersburg. In Moscow, an estimated 15,000 demonstrators gathered in and around Pushkin Square in the city center, where clashes with police broke out and demonstrators were roughly dragged off by helmeted riot officers to police buses and detention trucks. Some were beaten with batons. Navalny’s wife Yulia was among those arrested. Police eventually pushed demonstrators out of the square. Thousands then regrouped along a wide boulevard about a kilometer (half-mile) away, many of them throwing snowballs at the police before dispersing. Some later went to protest …