Open Russia, financed by Kremlin critic Mikhail Khodorkovsky, said the searches were taking place as part of an old criminal investigation into Yukos, a defunct oil giant once owned by Khodorkovsky.

A grouping called the United Democrats is backing hundreds of opposition activists running in local elections this Sunday, hoping to get a toehold in regional politics - dominated by the ruling United Russia party.

More than 20 searches were carried out in the regions of Vladimir, Ivanovo, Tatarstan and Novgorod at the group's offices as well at the homes of activists running in the elections, campaign staff and election monitors, Open Russia said.

Police also confiscated equipment and opposition newspapers from Open Russia's office in Moscow, and searched the Moscow editorial office of MBKh, a media outlet financed by Khodorkovsky, the group said.

"It's obvious the independent candidates and United Democrats' offices are not linked to Yukos; this is an act of pressure and lawlessness from the state in the interests of United Russia...," said Anastasiya Burakova, an Open Russia activist.

On Tuesday, an ally of leading opposition figure Alexei Navalny running for the city council in Novosibirsk said an assailant had burst into his group's campaign office and smashed a bottle on the floor containing a caustic and foul-smelling substance.

Russia's treatment of President Vladimir Putin's political opponents has come under international scrutiny since Navalny was airlifted to Germany last month to be treated for what German medics say was poisoning with a rare, military-grade nerve agent. Russia has said it has seen no solid evidence that he was poisoned.