Alexei Navalny, Russia’s most prominent opposition figure to President Vladimir Putin, has denied reports that he suffered an allergic reaction after falling ill in prison.
"I have never had an allergy. Not to food or pollen or anything else," Mr Navalny, 43, wrote in his blog.
He was taken to hospital with a swollen face, eye problems and rashes on his body on Sunday.
He had been in jail since Wednesday for calling unauthorised protests that took place over the weekend.
Mr Navalny, who is a fierce critic of the Kremlin, has since been discharged from hospital and has returned to jail, where he is serving the rest of a 30-day sentence.
"At night I woke up with a hot and prickly face, ears and neck," the activist said. "I felt like I'd had my face rubbed with glass wool."
"I had the thought, maybe I've been poisoned."
He said his doctors and lawyers were not given details of his illness.
Mr Navalny’s lawyer, Olga Mikhailova, said outside the hospital on Monday afternoon that her client was "poisoned by some unknown chemical substance".
The arrest of the activist was part of a huge police crackdown on protesters in Moscow.
Almost 1,400 people were detained in a police crackdown on Saturday's protest, the largest number in Moscow this decade, a Russian group that monitors police arrests said on Sunday
The anti-government protests were against the exclusion of independent candidates from the September 8 ballot for the Moscow City Council.
The rallies come amid wider public anger over declining living standards, which has damaged Mr Putin's approval ratings.