A doctor who visited Russian opposition leader Alexei Navalny says she cannot rule out he has been poisoned.
Mr Navalny was taken to hospital from prison on Sunday after suffering “severe swelling of the face and skin redness”, his spokeswoman said.
The dissident was jailed last Wednesday for 30 days after calling for unauthorised demonstrations in Moscow to protest against the exclusion of opposition candidates from local elections later this year.
Police acknowledged arresting more than 1,000 people during Saturday’s protests – one of the biggest crackdowns in recent years against the opposition.
Mr Navalny was transferred to hospital on Sunday morning after developing signs similar to that of an acute allergic reaction.
A doctor at the hospital treating him told the Russian news agency Interfax that Mr Navalny had been diagnosed with hives and was improving.
However, another doctor Anastasia Vasilyeva, who has treated him in the past and was able to speak briefly with him and look at him through the crack of a door on Sunday, said she could not rule out he was poisoned.
“We cannot rule out that toxic damage to the skin and mucous membranes by an unknown chemical substance was inflicted with the help of a ‘third party’,” the doctor said.
Ms Vasilyeva reported Mr Navalny had a rash on his upper body, skin lesions and discharge from his eye.
His spokeswoman Kira Yarmysh said it wasn’t clear what had caused the allergic reaction and that Mr Navalny had never suffered from a similar reaction in the past.
“It’s still unclear what’s wrong with him, but it looks strange,” his lawyer, Olga Mikhailova, wrote on Facebook on Sunday evening.
She said he had developed “an acute allergic reaction to an unknown allergen” and stressed that Mr Navalny had no known allergies.
Meanwhile, Ms Vasilyeva has requested samples from his bed sheets, skin and hair to be tested.
She said she found it suspicious she had been blocked from being allowed to examine him properly, Reuters reported.
The hospital where his spokeswoman said he was being treated could not be reached for a response.
Mr Navalny has regularly criticised the Kremlin and described the country’s main party, United Russia, which is led by President Vladimir Putin, as a “party of crooks and thieves”.
The UK’s Foreign Office said: “We are concerned by the detention on 27 July of over a thousand protesters, including journalists and prominent opposition figures, a significant use of force during a peaceful demonstration in Moscow.
“The Russian government continues to disregard the rights of its people to freedoms of expression, association and peaceful assembly.
“We call for the Russian government to comply with its international commitments on human rights in the Council of Europe and the Organisation for Security and Co-operation in Europe.”
Authorities in Moscow say the opposition candidates were barred from taking part in city council elections because they had failed to collect enough genuine signatures backing them. The candidates say this claim is false.