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.
