MOSCOW: Russian opposition leader Alexei Navalny was “poisoned” by an unidentified toxic substance but doctors have sent him back to jail despite his condition, his lawyer and the personal physician said Monday.

President Vladimir Putin’s top opponent was rushed to hospital on Sunday a day after almost 1,400 people were arrested at an unauthorized protest, in the largest police crackdown on dissent in recent years.

Navalny was hospitalized following what was described as an acute allergic reaction, but his supporters later said they believed he had been exposed to poison.

“It is indeed poisoning by some unknown chemical substance,” his lawyer Olga Mikhailova told reporters Monday outside Moscow’s hospital No 64, which treated Navalny. Navalny on Sunday had swollen eyelids, discharge in the eye and a rash on his upper body, his personal physician Anastasia Vasilyeva said. She told reporters on Monday that both she and the 43-year-old politician believe the reaction could have been a response to “some chemical agent”.