Russian prime minister Vladimir Putin has accused US secretary of state Hillary Clinton of encouraging and supporting the election protesters and warned of a wider crackdown on unrest.
By describing Russia's parliamentary election as rigged, Mr Putin said Mrs Clinton "gave a signal" to his opponents.
"They heard this signal and with the support of the US state department began their active work," Mr Putin said on TV.
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 in Sunday's parliamentary election.
The demonstrations have been some of the biggest and most sustained protests Russia has seen in years, and police have detained hundreds of protesters.
Mr Putin's United Russia party barely held on to its majority in parliament, with official results giving it about 50% of the vote, down from 64% four years ago.
But the fraud allegations indicate that support for United Russia was even lower as Russians are growing weary of Mr Putin and his party after nearly 12 years in office.
Mr Putin warned that Russian authorities 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.
Moscow has already put out 50,000 police and troops on the streets, backed by water cannon.
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, with its website hit by hackers.