Alexei Navalny addresses supporters during a rally in Moscow. Kirill Kudryavtsev / AFP

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 personal physician said on Monday.

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

Navalny was hospitalised 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".

Navalny has been in jail since last week for calling an unauthorised rally.

Vasilyeva said doctors at the hospital diagnosed him with a skin condition, adding that he had improved after being treated with a steroid.

But he has been sent back to jail despite needing to continue the treatment, she said, raising concerns over the danger to his health in prison and the possibility that his cell could be contaminated.

A senior doctor at the hospital told reporters that Navalny's health was not under threat.

But Vasilyeva accused the hospital doctors of not wanting to find out what caused Navalny's condition.

"Alexei has been clearly sent back to the detention centre on orders from above," she said.

Vasilyeva is an ophthalmologist who had treated Navalny previously after he nearly lost the sight in one eye after an attack in 2017.

He has never suffered from allergies in the past, she said.

Agence France-Presse