MOSCOW (AP) — Moscow police on Friday raided the offices of Foundation for Fighting Corruption, an organization founded by Russian opposition politician Alexei Navalny, reportedly in connection to a slander case opened against him.

Last month, Russia's Investigative Committee launched a probe into Navalny on the charges of defamation, after the politician slammed people featured in a video promoting the constitutional reform extending President Vladimir Putin's rule as “corrupt stooges,” “people without consciousness" and “traitors.”

The Investigative Committee maintained that Navalny's comments “denigrate (the) honor and dignity” of a World War II veteran featured in the video.

The search of the politician's foundation on Friday took place when Navalny was being interrogated at the Investigative Committee in connection with the case. During the interrogation, investigators barred him from leaving Moscow, Navalny said in a Tweet.

“All this has been invented so that I couldn't travel to the regions,” campaigning against candidates from the ruling United Russia party ahead of the regional elections in September, the politician wrote.

He added that after the interrogation investigators took him to two more raids — of the apartment Navalny rents in Moscow and another apartment where he is registered as a resident.

The constitutional reform allowing Putin to stay in power until 2036 was approved earlier this month by 78% of voters in a week-long nationwide ballot. Kremlin critics condemned the reform as undemocratic and pointed to numerous reports of violations and suspected incidents of voter fraud during the plebiscite.

Navalny actively criticized the reform, but didn't advocate for voting against it, saying the results of the ballot would be rigged anyway. He urged his supporters to rather focus on regional elections in September and fight against candidates put forward by United Russia in 31 Russian regions.

“It is a real fight in which a real victory is possible,” Navalny said.