Prime Minister Vladimir Putin strongly criticized U.S. Secretary of State Hillary Rodham Clinton on Thursday for encouraging and supporting Russians protesting election fraud, and warned of a wider crackdown on dissent.
By describing Russia's parliamentary election as rigged, Putin said, Clinton "gave a signal" to his opponents.
"They heard this signal and with the support of the U.S. State Department began their active work," Putin said in televised remarks.
Clinton has repeatedly criticized Sunday's parliamentary vote in Russia, saying "Russian voters deserve a full investigation of electoral fraud and manipulation."
Russian protesters have taken to the streets in Moscow and St. Petersburg for three straight nights despite a heavy police presence, outraged over observers' reports of widespread ballot box stuffing and manipulations of the vote count. This week has seen some of the biggest and most sustained protests Russia has faced in years, and police have detained hundreds of protesters.
Thousands were expected to join protests in Moscow and other cities on Saturday.
Putin's United Russia party barely held onto its majority in parliament, with official results giving it about 50 per cent of the vote, down from 64 per cent four years ago. But the fraud allegations indicate that support for United Russia was even lower than that, and Russians appear to be growing weary of Putin and his party after nearly 12 years in office.
Wider protests planned on Facebook
Moscow has already put about 50,000 police and 2,000 paramilitary troops on the streets, backed by water cannon.
Putin warned that the government might take an even harder line.
"We need to think about strengthening the law and holding more responsible those who carry out the task of a foreign government to influence our internal political process," he said.
Russia's only independent election monitoring group, which is supported by grants from the United States and European governments, has come under heavy official pressure in recent weeks. The Golos website documenting violations was hacked and the group was fined the equivalent of $1,000 after prosecutors accused it of violating election law.
Also Thursday, Russia's top election official urged prosecutors to study photographs and video clips circulating on social networking sites that purport to show election fraud, and signalled that those who posted the materials could be punished.
If the images show genuine violations, they will be investigated, Central Election Commission chief Vladimir Churov said. But if evidence is found that the photographs and videos were "provocations" or faked, "those who created, commissioned or sponsored them will be held to account, he said.
Opposition groups have called for a mass protest near the Kremlin on Saturday. More than 26,000 people have signed up to a Facebook page on the protest.
A map circulating on the internet shows protests planned for Saturday in more than 75 cities around Russia, while a page on LiveJournal lists more planned anti-vote fraud protests in 15 countries around the world.