MOSCOW—A Moscow court on Saturday rejected Russian opposition leader Alexei Navalny’s appeal against his prison sentence, even as the country faced a top European rights court’s order to free the Kremlin’s most prominent foe. Speaking before the verdict, Navalny urged Russians to stand up to the Kremlin in a fiery speech mixing references to the Bible and “Harry Potter.” A lower court sentenced Navalny earlier this month to two years and eight months in prison for violating terms of his probation while recuperating in Germany from a nerve agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation. Navalny, 44, an anti-corruption crusader and President Vladimir Putin’s most vocal critic, appealed the prison sentence and asked to be released. The Moscow City Court’s judge on Saturday only slightly reduced his sentence to just over 2 1/2 years in prison, ruling that a month-and-half Navalny spent under …