Russian opposition leader Alexei Navalny was moved back to jail from a hospital Monday even though his physician raised suspicions of a possible poisoning after he suffered facial swelling and a rash while in custody.

Details about Navalny’s condition were scarce after he was rushed to a hospital Sunday with what authorities said was a suspected allergy attack inside a detention facility where he was serving a 30-day sentence for calling an unsanctioned protest. The 43-year-old political foe of President Vladimir Putin was arrested several days before a major opposition rally Saturday that ended with nearly 1,400 people detained.

Tensions are running high in Moscow as dozens of protesters remain in custody and the opposition called for a new rally Aug. 3.

In a blog post written in detention, Navalny said he may have been exposed to an unknown chemical agent while in custody. Navalny recalled how 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.’”

Dr. Anastasiya Vasilyeva, who has been Navalny’s physician for several years, visited him Monday shortly before he was discharged from the hospital and sent back to the detention facility even before the necessary tests were run on him.

Doctors at the hospital initially said Navalny had a severe allergy attack, but Vasilyeva said that the swelling and the rash on his face could be consistent with chemical poisoning. She said the incarceration would jeopardize his health.

“He has not fully recovered. He should have been left under medical supervision,” she told reporters outside the hospital, adding that the doctors didn’t even try to determine what caused the swelling and rash. “Who is going to watch over him at the detention facility? They are not qualified to provide him with professional help.”

Vasilyeva expressed concern that the chemical agent that caused the outbreak could still be in his prison cell.

Navalny’s attorney, Olga Mikhailova, told reporters earlier the outbreak was caused by “poisoning, by some kind of chemical substance” but that its source wasn’t established. She said he has been given anti-inflammatory steroids and that the swelling subsided.

Navalny said Monday he felt and looked better now — “like someone who’s been drinking for a week.” His face is visibly swollen in the picture he attached to the blog post, with red circles around the eyes.

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

Although there has been no confirmation that Navalny was poisoned, suggestions by his doctor that he was exposed to some kind of toxic chemical in jail raised suspicions among his supporters of possible foul play. Some Kremlin political opponents have been poisoned or killed in recent years, although Russian officials denied any involvement.

“Are they such idiots to poison me in the place where they could be the only suspects?” Navalny wrote in the post, referring to the Russian government. “There’s only one thing I can say with certainty: Power in Russia is in the hands of the guys who really are stupid.”

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 of 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.

He has been attacked several times. In 2017, an assailant doused him with a green antiseptic, and Navalny sustained a chemical burn in one of his eyes, which left to a partial loss of vision. Navalny was able to travel abroad for treatment.

On Saturday, baton-wielding police wrestled with protesters in what might have been the largest unsanctioned protest in Russia in a decade.

Putin, who was out of town to lead a naval parade in St. Petersburg on Sunday, has not commented on the massive protests.

Opposition activists as well as ordinary Muscovites vented their anger over officials’ decision to exclude a dozen independent candidates from the ballot for a Sept. 8 election of the Moscow city legislature, which is dominated by the ruling, pro-Kremlin party.

The candidates’ supporters had earlier picketed the headquarters of the Moscow Election Commission and rallied on a central square for several days straight. After authorities claimed that some of the 5,500 signatures each candidate collected were forgeries, the candidates went to the local election commission to protest, some bringing the same people whose signatures were ruled invalid.

Among those taken into custody Saturday were several would-be candidates. One of them, Ilya Yashin, was sentenced Monday to 10 days in jail for calling the protest. Another, Dmitry Gudkov, is due to appear in court on Tuesday.

Andrei Pertsev of the Moscow Carnegie Center said in an opinion piece last week that the Kremlin was not going to allow opposition candidates on the ballot from the start, fearing their presence in the local council, however low-key, could give them a platform for campaigning for the next parliamentary election.

“It was a matter of principle for the Kremlin not to allow the opposition candidates on the ballot,” he said, adding that a Moscow city lawmaker would be an “odds-on favorite” to win a seat at the 2021 parliamentary election.

Over the weekend, the European Union condemned what it called “the disproportionate use of force against peaceful protesters.” The EU said the weekend actions already came in the wake of “the worrying series of arrests and police raids against opposition politicians” in recent days.

(AP)