Police detained Denis Bilunov, the executive director of the United Civic Front, at a train station on charges of possessing counterfeit money.
Bilunov traveled from Moscow to help organize a so-called March of Dissent rally on the sidelines of a two-day European Union-Russia summit that starts today in Samara.
Protest organizers say police are carrying out an intimidation campaign against opposition activists ahead of the rally.
Separately, opposition protestors said they plan to intensify their campaign against President Vladimir Putin by holding a march in St. Petersburg during an international economic conference in June.
Former world chess champion Garry Kasparov, an opposition leader, says as many as 5,000 people could turn out for the June 9 protest in St. Petersburg.
Opposition activists say they are also planning protests in Moscow on June 11 and in Voronezh in early June.
(ITAR-TASS, RIA Novosti, RFE/RL Central Newsroom)