Russian opposition leader Alexei Navalny has been discharged from a Moscow hospital where he was being treated for an unexplained health problem and returned to prison under guard, Anastasia Vasilyeva, his doctor, says.

Separately, Navalny's lawyer Olga Mikhailova told reporters in front of the hospital that her client had been poisoned by an unidentified chemical agent, a thesis put forward by Vasilyeva a day earlier.

Navalny, 43, was rushed to hospital from jail on Sunday morning with what his spokeswoman said were signs of an acute allergy with "severe swelling of the face and skin redness."

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.

AAP

On Monday, Ms Vasilyeva wrote on Facebook that "some toxic agent may be the reason for Alexei Navalny's 'illness'".

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

After his hospitalisation on Sunday doctors diagnosed Mr Navalny with a skin condition, Ms Vasilyeva said.

READ MORE Moscow police arrest more than 1000 at rally calling for fair elections

They treated him with a steroid and he got better, but he has been sent back to jail despite needing to continue the treatment, she told reporters.

She also said she suspected his prison cell could be contaminated.

Ms Vasilyeva is an ophthalmologist who had treated Mr Navalny previously for an eye injury.

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

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

Mr Navalny was jailed for calling a rally against authorities' move to block prominent independent candidates from taking part in Moscow parliament elections in September.