Russian Prime Minister Vladimir Putin is lashing out at the United States, accusing Washington of riling up the opposition.

Mr. Putin said Thursday that remarks by U.S. Secretary of State Hillary Clinton “gave a signal” to his opponents.

Clinton said Tuesday that Russia's parliamentary elections were neither free nor fair, and that the results raised “serious concerns.”

Opposition leaders have overseen three days of protests to express their frustration with the elections. International observers say Sunday's vote was marred by widespread allegations of ballot-box stuffing and other irregularities.

Former Soviet leader Mikhail Gorbachev has called for Russia's parliamentary election to be annulled and a new vote held. The 80-year-old said the results do not reflect the will of the people.

Fifty-thousand police officers backed by helicopters patrolled the streets of Moscow Wednesday – arresting 50 demonstrators – in a show of force. Security officials said 550 people had been arrested in Moscow protests the night before.

A social networking site is promising nightly protests, but Russia's state-run television channels have not mentioned the protests or the police buildup.

Opposition organizers have scheduled four major protests to be held in Moscow over the next 10 days, with the first set for Saturday near the Kremlin. Authorities have granted a permit for 300 people, but 23,000 have pledged to attend through Facebook and a Russian social networking site.

Pro-government supporters also rallied around the Kremlin. Prime Minister Vladimir Putin's United Russia party won Sunday's vote, but with a considerably reduced parliamentary majority, at around 50 percent.