Navalny was sentenced to 20 days in prison on Monday, so he would not be able to lead a major rally he had planned to stage in Putin's hometown of St. Petersburg on Saturday.
"[My sentence] is a gift to Putin for his birthday," The Guardian quoted Navalny, as quipping in a Moscow courtroom.
Navalny was detained on Friday, while making plans to travel to a rally in a provincial city. He was accused by the police of repeatedly violating a law on organising public meetings, which is punishable by up to 30 days in jail.
Navalny has said he would like to stand for president in the election next March but the electoral authorities have said he is not eligible because he is serving a suspended sentence for fraud.