Alexei Navalny

Officials gave no details of his condition but Mr Navalny's spokeswoman said he had had an allergic reaction with severe facial swelling - something she said he had not experienced before.

A hospital source told Russian media his condition was satisfactory,

An eye doctor who treated Mr Navalny previously is concerned he may have been injured by a chemical substance.

Mr Navalny was jailed for 30 days last week after calling for unauthorised protests, which took place on Saturday.

Thousands of people joined rallies nationwide, calling for the resignation of Prime Minister Dmitry Medvedev over corruption allegations.

At least 500 other protesters were detained in the capital and across the country.

Most of the marches were illegal, organised without official permission.

TV pictures showed demonstrators chanting "Down with [Russian President Vladimir] Putin!", "Russia without Putin!" and "Putin is a thief!".

Correspondents say the marches appear to be the biggest since anti-government demonstrations in 2011/2012.