Alexei Navalny, Russia's most prominent opposition figure, has questioned reports he suffered an acute allergic reaction after being taken ill in jail.
"I have never had an allergy. Not to food or pollen or anything else," he wrote in a blog post.
The 43-year-old was taken to hospital with a swollen face, eye problems and rashes on his body. He was in jail for calling for unauthorised protests.
His doctor suggested he might have been exposed to "some toxic agent".
Mr Navalny has since been discharged from hospital and has returned to jail.
What did Alexei Navalny say?
In a statement in Russian posted on his blog and written from his cell, Mr Navalny said he had never had an allergic reaction in his life. He added that his wife suffers from allergies, meaning he knows what they look like.
"At night, I woke up with a hot and prickly face, ears and neck," he wrote. "I felt like I'd had my face rubbed with glass wool."
"I had the thought, maybe I've been poisoned."
Mr Navalny was sent to the hospital in the morning, where the doctors diagnosed him with "contact dermatitis".
Mr Navalny wrote in his blog post that his lawyers and doctors were not given his diagnosis, and that a detailed report emerged about his diagnosis on Russian news agency Interfax that he himself had not been told.
While he did not outright deny the possibility of an allergic reaction, Mr Navalny said police at his hospital room door were acting "like they had something to hide".
He said he was sure the local police did not poison him, as "they were shocked by the sight of me more than I was". But he suggested the Russian authorities were "stupid" enough to do so.
Mr Navalny's doctor Anastasia Vassilieva earlier told AFP news agency it was "absurd to call it an allergy". He "needs to be under close medical supervision", she said, and should be allowed to call his relatives.
In a Facebook post, Ms Vassilieva had complained that she had been barred from visiting Mr Navalny. But based on what she saw of him through a door, she said he may have been injured by a "chemical substance from a third person".
Before Mr Navalny was released from hospital, his medical team said they were able to gain access to him and had arranged for samples of his hair and T-shirt to be tested independently.
Mr Navalny was jailed for 30 days last week after calling for unauthorised protests, which took place on Saturday.
Nearly 1,400 people were detained during the demonstrations against the exclusion of opposition candidates from local elections.
'A thorn in the Kremlin's side'
Alexei Navalny is Russia's most prominent opposition activist - and one of President Putin's most vocal critics.
That's why news of a sudden illness makes headlines.
Especially if it's a sudden illness contracted in a Moscow jail.
There's been no confirmation that Mr Navalny was poisoned. But his doctors - and his supporters - are keen to know what sparked such sudden symptoms.
He is no stranger to health scares. Two years ago Mr Navalny was the victim of an assault. He suffered a chemical burn in his right eye after someone threw green-coloured antiseptic in his face.
He is a constant thorn in the Kremlin's side. The authorities know that, with President Putin's personal ratings falling, the charismatic anti-corruption activist has the ability to mobilise anti-Kremlin sentiment.
But it's too early to conclude that his current state of health is connected to his political views or activities.
Media reports said about 20 people, including journalists, were also detained after gathering outside the prison hospital on Sunday night where Mr Navalny was being treated.
Russian media report that most of the activists detained over the weekend have now been released.
However, about 150 were still in custody on Monday. They are facing judicial hearings and may be charged in connection with the unauthorised rally.
Who is Alexei Navalny?
The 43-year-old made his name in Russia as a grassroots anti-corruption campaigner.
He led the country's biggest street protests against President Putin in 2011 and has repeatedly been jailed, usually for his involvement in unauthorised demonstrations.
Mr Navalny suffered a serious chemical burn to his right eye in 2017 after he was assaulted with antiseptic dye.
He attempted to stand in last year's presidential race but was barred because of previous fraud convictions in a case he says was politically motivated.
What happened during the protests?
Thousands of Russians took to the streets last Saturday to demand fair elections. The demonstrations came after 30 opposition candidates were barred from standing in local races this September.
Officials said the candidates had failed to collect enough valid signatures to stand, but opposition groups argued that the barring was politically motivated.
Mr Navalny helped to organise the demonstrations. Officials said they had arrested nearly 1,400 people - making it one of the biggest crackdowns in recent years.
Images from Saturday showed police in riot gear pushing crowds from the mayor's office in central Moscow.
A number of protesters could be seen bleeding, while at least two members of the security forces reportedly received eye injuries from pepper spray.
The Moscow's mayor office has given official approval for a further protest on 3 August, according to a report in the independent Novaya Gazeta newspaper.