Kremlin critic Alexei Navalny was barred Friday from leaving Moscow as police carried out a new raid on his offices.
Russia's opposition has accused the authorities of cracking down on opponents to President Vladimir Putin after he oversaw a vote this month allowing him to potentially remain in power until 2036.
Navalny said he had been questioned by investigators and banned from leaving the capital as part of a new criminal case against him.
Law enforcement officers searched the offices of his Anti-Corruption Fund (FBK) in southern Moscow while he was being interrogated.
The 44-year-old claimed the ban was an attempt to prevent him from travelling the country to promote a tactical voting strategy to oppose pro-Putin candidates in more than 30 regional elections in September.
"This wretched regime, we have to fight it," Navalny said in a video address on Instagram.
"Take part in the smart voting campaign to crush the swindlers and thieves that have seized power in our beautiful country."
'Putin's ratings have crashed'
Pro-Kremlin politicians suffered losses in Moscow city polls last year after the opposition leader called on supporters to vote tactically.
He has called on Russians to use the same strategy to oust members of the increasingly unpopular ruling party United Russia in the regional ballots.
Navalny said he was questioned by investigators as part of a new probe into suspected slander over comments he made on social media.
He said 15 investigators would be involved in the probe.
In June, the Investigative Committee, which probes major cases, said Navalny was suspected of defaming a World War II veteran.
The trained lawyer said in the video that law enforcement also planned to search two apartments, including one where he lives.
Police have repeatedly raided Navalny's offices, and the Kremlin critic has personally been the target of multiple criminal probes.
He was barred from challenging Putin in a presidential election in 2018 over a prior conviction his supporters said was poltically motivated.
Navalny's ally and up-and-coming opposition politician Lyubov Sobol accused the Kremlin of tightening the screws on critics.
"Putin's approval ratings have crashed, the economy is in doldrums, thousands-strong protests in the regions," said Sobol, referring to anti-Kremlin rallies in the Far-Eastern region of Khabarovsk.
"What to do? Come search the FBK for the umpteenth time!" she wrote on Twitter.
Navalny says the seven-day vote on the changes to the constitution was not properly monitored and set a record for falsified ballots.
This week Moscow police detained nearly 150 people who marched in the city center against the constitutional reforms.