A Russian opposition-party leader and vocal Vladimir Putin critic has suffered a mysterious “allergic reaction” days after being jailed for organizing protests, according to a report.

Alexei Navalny, 43, was arrested in a sweep of opposition candidates Wednesday for calling for Saturday demonstrations in Moscow, after electoral authorities banned several insurgents from taking part in an upcoming election.

But Kyra Yarmish, a spokesperson for the jailed politician, revealed on Sunday that Navalny developed “severe swelling of the face and redness of the skin,” according to a tweet translated by CNN.

The cause was unclear, and Navalny had never experienced an allergic reaction before, according to the spokeswoman, who added that Navalny was being treated at a hospital under police protection.

Some 800 people were arrested in Moscow on Saturday amid fraught protests over the electoral commission authority’s decision to bar 30 candidates from a Sept. 8 election, because they failed to gather the 5,000 petition signatures necessary to get onto ballots.

The opposition candidates say the petitions are a sham, because citizens who support anti-Putin candidates run the risk of being arrested.

Navalny tried to challenge Putin last year, but was barred from running over a fraud conviction he says was a political hit job.