Moscow: Russian opposition leader Alexei Navalny has been hospitalised after suffering an acute allergic reaction which one doctor said may have been the result of him being poisoned with an unknown chemical substance.

Navalny, 43, was rushed to hospital from jail where he is serving a 30-day sentence for violating tough protest laws, a day after police in Moscow detained more than 1000 people for an illegal demonstration called for by Navalny.

Russian opposition activist Alexei Navalny films himself in a police station in Moscow, Russia, at the time of his arrest last week. Credit:Instagram via AP

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

A doctor at the hospital treating him told the Interfax news agency that Navalny had been diagnosed with hives and that he was feeling better.