A court in Moscow has rejected jailed Russian Opposition Leader Alexei Navalny's appeal for early release on health grounds, in the wake of him suddenly falling ill while in custody and being rushed to hospital.

Key points: Mr Navalny was in prison for calling an unsanctioned protest

Mr Navalny was in prison for calling an unsanctioned protest He was rushed to hospital after suffering facial swelling and a rash

He was rushed to hospital after suffering facial swelling and a rash His doctor says Mr Navalny may have been poisoned and he should be in hospital

Mr Navalny was moved back to prison from a hospital on Monday (local time) even though his physician raised suspicions he was poisoned, leading to facial swelling and a rash.

Details about Mr Navalny's condition were scarce after he was rushed to a hospital on the weekend with what authorities said was a suspected allergy attack while serving a 30-day sentence for calling an unsanctioned protest.

In a blog post written in detention, Mr Navalny said he had never had allergies and may have been exposed to an unknown chemical agent.

He said his face started to become swollen on Saturday and it worsened the next day.

"I got up in the morning and when my cellmate saw me he said 'you need to see a doctor now'," his post read.

The 43-year-old political foe of President Vladimir Putin was arrested several days before a major opposition rally that ended with nearly 1,400 people detained.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 47 seconds 47 s Police clash with protestors in Moscow city square

Anastasiya Vasilyeva, Mr Navalny's physician of several years, said Mr Navalny was sent back to the detention facility before necessary tests were run on him.

Doctors at the hospital initially said Mr Navalny had a severe allergy attack, but Ms Vasilyeva said the swelling and the rash on his face could be consistent with chemical poisoning.

"He has not fully recovered, he should have been left under medical supervision," she told reporters outside the hospital, adding incarceration would jeopardise Mr Navalny's health.

Ms Vasilyeva expressed concern a chemical agent which caused his illness could still be in his prison cell.

'Poisoning, by some kind of chemical substance'

Russian Opposition Leader Alexei Navalny, is a lawyer and anti-corruption activist. ( AP: Evgeny Feldman/Navalny Campaign )

Mr Navalny's lawyer Olga Mikhailova earlier told reporters the sickness was caused by "poisoning, by some kind of chemical substance" but its source was not established.

She said Mr Navalny had been given anti-inflammatory steroids and the swelling had subsided.

Mr Navalny's face was visibly swollen in the picture he attached to the blog post and there were red circles around his eyes, but he said he felt and looked better now.

He said he would like to see CCTV footage to check if anyone entered his cell while he was on a walk, saying he had his own linen and toiletries and could not think of a possible cause unless someone left something near his bunk.

Some Kremlin political opponents have been poisoned or killed in recent years, although Russian officials have denied any involvement.

Mr Navalny, a lawyer and anti-corruption activist, has been the Kremlin's most formidable foe since 2011, when he led a massive wave of protests against Mr Putin and his party.

He has since been convicted on two sets of criminal charges, largely regarded as politically motivated, and spent numerous stints in jail for disturbing public order and leading unsanctioned protests.

Reuters/AP