MOSCOW — A day after an unauthorized election protest he planned drew mass arrests in Moscow, Aleksei A. Navalny, Russia’s most prominent opposition leader, was hospitalized with a “severe allergic reaction” in jail, his spokeswoman said on Sunday.

“Over his whole life, Aleksei has never experienced an allergic reaction,” the spokeswoman, Kira Yarmysh, wrote on Twitter. But she said that his face was severely swollen and he had red spots on his skin.

Mr. Navalny was arrested on Wednesday and sentenced to 30 days in jail for calling a rally on Saturday to protest a decision by the election authorities to bar several opposition candidates from running for Moscow’s City Council.

The protest led to the arrests of more than 1,300 demonstrators. The rally was the latest in a series of street demonstrations staged as President Vladimir V. Putin’s approval ratings have dropped amid economic hardship.