Alexei Navalny, Russia’s most prominent opposition leader, was poisoned in jail by “an unknown chemical substance”, according to his lawyer, Olga Mihailova.

The Kremlin critic was discharged from hospital on Monday and returned back to prison under guard, his doctor said.

Mr Navalny was arrested as he left his home in Moscow on Wednesday, ahead of a demonstration calling for free and fair elections.

A judge sentenced him to 30 days imprisonment. Three days in, he was taken ill with what was described as a severe allergic reaction, and transferred to hospital.

Allies of the opposition leader said authorities acted suspiciously, initially refusing to allow Mr Navalny’s doctor to examine him in hospital.

Writing on social media earlier on Monday, Dr Anastasiya Vasilyeva, who has been Navalny’s physician for several years,​ said her patient was displaying clear signs of intoxication by an unknown chemical, but that state medical staff had not sent samples off for necessary blood tests.

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Later, the doctor told Russian media that she had managed to obtain hair samples and a T-shirt to be sent away for analysis.

“We will do independent checks, perhaps even send them away to Europe,” she said.

On Sunday, Leonid Volkov, Mr Navalny’s chief of staff, said there were no immediate signs of a “conspiracy”. He revealed he had himself fallen ill after a short stay in the same cell, and suggested that sanitary norms were not being properly observed.

Allies say Mr Navalny risks another flare-up if traces of the substance they fear poisoned him are still present in his cell.

Nearly 1,400 people were arrested during Saturday’s protest – one of the largest protests Moscow has seen for seven years. The Kremlin has made little secret of its desire to turn the screws on Russia’s opposition. It has conducted night-time raids on most of the excluded opposition candidates, and warned of the prospect of a criminal investigation.

Tensions remained high in Moscow on Monday, as dozens of protesters remained in custody and the opposition called for a new rally over the weekend.

Mr Navalny has himself been arrested on multiple occasions over trivial violations of controversial protest laws, spending every seventh day of the past calendar year in police custody.