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 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.

‘Rotten system’

The Russian opposition leader is serving a 30-day jail sentence for calling Saturday’s rally after authorities blocked prominent independent candidates from taking part in Moscow parliament elections in September.

The Kremlin on Monday scrambled to contain a fresh political crisis after police brutally crushed a peaceful rally that saw thousands take to the Moscow streets at the weekend to urge the authorities to open up the tightly-controlled system.

Ahead of the rally police detained a number of popular opposition politicians who have fought to get on the ballot.

On Monday, prominent opposition politician Ilya Yashin was jailed for 10 days, while a lesser known independent would-be candidate, Konstantin Yankauskas, was sentenced to seven days in jail.

“I am not afraid of your rotten system, your arrests or your detention centres,” Yashin said in court, according to an account that he published on social media.

The opposition has called for a new protest on Saturday and more than 8,000 people on Facebook indicated interest in the fresh rally.

Independent politician Yulia Galyamina said authorities had apparently sought to prevent a new rally from taking place by jailing top activists.

Despite the crackdown, thousands of protesters — many in their 20s and early 30s — on Saturday turned up in central Moscow and clashed with police.

Liberal Russian media estimated the turnout was up to 15,000 people.

Germany slammed “the disproportionately tough police action” and urged the detained protesters’ release.

On social media, Russians have seethed with anger over the brutality of police tactics, sharing stories of injuries and detentions.

As police on Saturday beat up protesters with batons and indiscriminately arrested people including the elderly and passers-by, Putin dived to the bottom of the Gulf of Finland aboard a submersible.

Activists ‘banged up’

The Bell, a respected Russian-language online publication, said the Kremlin had underestimated Muscovites’ anger and readiness to protest.

“Now we’ll have to think what to do with all of this,” it quoted a Kremlin source as saying.

A person familiar with the Kremlin thinking told the Bell that the most prominent activists would be “banged up”, while the rest would be intimidated.

More than 21,000 people signed a petition calling on Moscow Mayor Sergei Sobyanin to resign. (AFP)

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