Doctors for Russia's most prominent opposition figure, Alexei Navalny, are conducting tests to figure out the cause of his sudden illness.
Mr Navalny fell ill in jail on Sunday. Initial reports suggested that he had suffered an acute allergic reaction, resulting in a swollen face, eye problems and rashes on his body.
However, Mr Navalny's personal doctor said on Sunday he'd never suffered from allergies before, and that he may have been exposed to "some toxic agent".
His medical team said they were able to gain access to him on Monday, 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.
The European Union criticised the "disproportionate" use of force against the protesters, saying it undermined the "fundamental freedoms of expression, association and assembly".
'A thorn in the Kremlin's side' Analysis box by Steve Rosenberg, Moscow correspondent
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.