Raids began at 6:00 am at about 30 places: Opposition leader Alexei Navalny said

Russia's opposition said on Tuesday that investigators had launched raids on its offices across the country, in the latest move to put pressure on government critics.

"They started at 6:00 am and are taking place in at least 30 addresses," opposition leader Alexei Navalny said on Twitter.

"The 'law enforcement system of Russia' is using all of its efforts to protect corrupt officials and bribe takers."

Ivan Zhdanov, the director of Navalny's Anti-Corruption Foundation (FBK), said the raids were taking place in 15 cities and that investigators were outside the foundation's offices in Moscow.

They come less than a week after Russian authorities declared the FBK a "foreign agent", the latest move to increase pressure on the opposition.

Authorities claim that FBK, which often publishes investigations into top state officials, receives funding from foreign organisations, a claim it denies.

Mass raids were carried out at dozens of opposition offices last month, shortly after municipal elections that saw Kremlin allies suffer significant losses.