MOSCOW — In a nationwide crackdown to prevent discontent in Moscow from spreading to far-flung regions, Russian security forces on Thursday raided and searched hundreds of homes and offices of activists across the country who were affiliated with the opposition leader Aleksei A. Navalny.

The raids, the biggest operation yet against Mr. Navalny and his supporters, were carried out in more than 40 cities and towns as part of a criminal money-laundering investigation announced in August by the authorities against his Anti-Corruption Foundation. The foundation has been the vanguard of recent street protests in Moscow that led to the arrests of more than 2,000 people.

Mr. Navalny issued a statement denouncing the raids as the biggest police operation, in geographical reach, in Russia’s modern history and a clear attempt to “intimidate” and “demoralize.” He said security officials from the police, the National Guard and the Federal Security Service, or F.S.B., had simultaneously raided more than 200 sites in 41 towns and cities.

Also raided as part of the same investigation were the homes of activists in Golos, an independent organization that monitors elections.