Moscow (CNN) Russian opposition leader Alexei Navalny, who was hospitalized with an "acute allergic reaction" after being arrested last week, may have been affected by an "unknown chemical substance," said a spokesman for Navalny, citing a physician.

Kira Yarmysh, Navalny's press representative, quoted a Facebook post by a personal doctor who was able to observe Navalny at the hospital where he was taken from a Moscow detention center.

"We cannot rule out toxic damage to the skin and mucous membranes by an unknown chemical substance with the help of a certain 'third person,'" the post read, adding that visiting doctors were not given full access to Navalny.

In 2017 Navalny was attacked with an antiseptic green dye that damaged vision in one of his eyes.

Russia's most outspoken opposition figure was detained by police Wednesday following a sweep of opposition candidates in the days leading up to anti-government demonstrations in the capital on Saturday.

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