"The cowardly loafers in black hats are running around our offices again," he wrote in a statement on his website on October 15.

Navalny, 43, has been one of President Vladimir Putin's most vocal critics for the better part of a decade, enduring multiple incarcerations, a barred attempt to run for president, and a hamstrung bid for the Moscow mayor's post.

OVD-Info, a group that monitors protests and arrests in Russia, said on October 15 that Navalny's offices in Ufa, Saratov, Samara, Yekaterinburg, Yaroslavl, Chelyabinsk, and Novokuznetsk were being searched by law enforcement.

FBK director Ivan Zhdanov wrote on Twitter that Navalny's offices in Belgorod, Voronezh, Izhevsk, Kemerovo, Vladivostok, Cheboksary, and Stavropol , were also searched.

The online human rights news resource Mediazona and OVD-Info also reported that the homes of Navalny's associates across Russia were searched the same day.

"The nature of the searches of the supporters of Navalny and FBK indicates only one goal: to cause maximum material damage," Zhdanov said. "Previously, there was still a goal to intimidate, but this does not work."

The Investigative Committee said in a statement that it conducted searches in 30 regions as part of a "preliminary investigation" and that some of the employees of the offices were summoned for questioning.

"Items and documents relevant to the investigation were recovered," it said, giving no further details.

Navalny said that police broke the door in his office in Moscow "for the fourth time in the last two months," and called the case against his foundation "completely fabricated."

Leonid Volkov, a project manager at FBK, described the searches on Telegram as "a terrorist act" conducted by the authorities.

Media reports say that some current and former associates of Navalny were detained for questioning in Moscow, Yekaterinburg, and Vladivostok.

There were no official statements regarding the reported searches and detentions.

In August and September, police searched Navalny's offices in dozens of cities across Russia.

The Investigative Committee said earlier that a probe had been launched into suspected money laundering by Navalny's foundation.

Navalny and his team have rejected the allegation, saying that the authorities were trying to cut off financial support to the foundation's branches.

In his October 15 statement, Navalny mocked the investigation, saying the authorities blocked one of the foundation's account so they could then send money to it from abroad to make it look as though FBK was being financed by foreigners.

"Soon, having seized all of the documents we have, they will then hire some people to work with us. From here they will release revelations from these people, make payments from accounts, and investigate the purpose of these payments," he said.

"And then, once this is done, they will reward each other and receive extraordinary titles for such excellent work," he added.

Last week, Russia's Justice Ministry branded the nonprofit FBK a "foreign agent" under a 2012 law that was widely criticized by the West for stigmatizing groups with the designation.

With reporting by Interfax