The leader of Russia's opposition Levy Front (Left Front) movement, Sergei Udaltsov, has been detained in Moscow.
Udaltsov's wife tweeted that her husband was detained on August 14 over July protests organized against the government's plan to increase the retirement age.
Tens of thousands of Russians rallied across the country on July 28 against the plan.
Udaltsov was one of the organizers of rallies in Moscow, where thousands rallied against the planned reform.
Legislation now under consideration by lawmakers would raise the retirement age to 65 for men by 2028 and 63 for women by 2034. Currently, the retirement age for men and women is 60 and 55 years, respectively.
Russian officials have warned for years that the pension age needs to be raised to take account the country's demographics, labor force, and projected budgets.
The plan sparked outrage across Russia after it was announced in June. Adding to criticism is the fact that the proposal was released by the cabinet on the eve of the opening of the World Cup.
Many critics accused the government of trying to slip the plan past Russians who were focused on the soccer tournament.
Putin's public-opinion ratings have slipped noticeably since the proposal was released. Last week, Putin attempted to tamp down the criticism, saying he would listen to "all opinions" on the matter.