Moscow -- Russian opposition leadersaid he was arrested on Wednesday, in an apparent move by the authorities to prevent a major protest rally this week.
In a video on his Instagram account, the anti-corruption campaigner said he was detained as he was leaving his Moscow home to go jogging and buy flowers for his wife on her birthday.
"People are right when they say that sport is not always good for your health," Navalny joked.
"I have been detained and am now at a police station wearing shorts like a stupid man," the 43-year-old said.
His spokeswoman, Kira Yarmysh, confirmed the arrest on her Twitter account.
Navalny's arrest came after more than 22,000 people rallied in the Russian capital on Saturday to demand free and fair local polls, incensed by the authorities' refusal to put popular opposition candidates on the ballot for the Moscow parliament elections in September.
The opposition says it was the largest protest since 2012 when tens of thousands rallied against election fraud during parliamentary polls.
Navalny threatened an even bigger rally this Saturday, near the mayor's office, unless Moscow's authorities register a new crop of popular politicians including Ilya Yashin and Lyubov Sobol within seven days.