Four days after being arrested by Russian authorities, Alexei Navalny, a longtime critic of President Vladimir Putin, has been hospitalized with what his spokeswoman has described as an "allergic reaction."

Navalny, one of the most prominent leaders of Russia's opposition movement, was arrested after calling for protests on Saturday against the exclusion of opposition candidates from city council elections in Moscow. He was sentenced to 30 days in prison.

The demonstration drew thousands of supporters. Russian police detained more than 1,300 people according to the , an independent monitor that tracks police departments in Russia.

On the day of his arrest, Navalny posted an Instagram video saying that police arrested him as he left his apartment to go for a jog and buy flowers for his wife's birthday.

Kira Yarmysh, Navalny's spokeswoman, wrote in a Twitter post that he had been hospitalized on Sunday morning with "severe swelling of the face and skin redness."

She said that the cause of Navalny's allergic reaction was unknown and that "he had never suffered from such reactions in the past." The Associated Press reported that according to Yarmysh, as of Sunday afternoon in Moscow, Navalny was in "satisfactory condition."

Election authorities barred opposition candidates from the upcoming election for Moscow's city council because they said the candidates didn't have enough valid signatures on nominating petitions. Candidates are required to collect about 5,000 signatures to run for election. The opposition candidates say they've been kept from the ballot for political reasons.

Early Saturday, Moscow police rounded up and detained several high-profile opposition politicians, including Ilya Yashin, Dmitry Gudkov and Ivan Zhdanov.

The 45-seat Moscow City Duma is controlled by the pro-Kremlin United Russia party. Those seats have a five-year term, and the entire council is up for reelection on Sept. 8.

NPR's Moscow correspondent, Lucian Kim, wrote in a tweet, "Reason for crackdown: Allowing even a few opposition politicians into Moscow city council would grant them legitimacy and exposure. And that could be a slippery slope to further erosion of Kremlin power."

Last week, Kim reported that Putin's approval rating has dropped in recent years. He noted, "The party has become so unpopular nationally that Putin ran for reelection as an independent last year. In upcoming Moscow city council elections, United Russia members have abandoned the party ticket and registered as independents."

The decision to bar some candidates from the city council election has triggered multiple protests across Russia this month.

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