Russian opposition leader Alexei Navalny was hospitalized on Sunday after suffering an acute allergic reaction that one doctor said may have been the result of him being poisoned with an unknown chemical substance.

Navalny, 43, was rushed to a hospital on Sunday morning from jail where he is serving a 30-day sentence for violating tough protest laws, a day after police in Moscow detained over 1,000 people for an illegal demonstration which Navalny called for.

His spokeswoman, Kira Yarmysh, said Navalny had signs of an acute allergy with "severe swelling of the face and skin redness."