Former Soviet leader Mikhail Gorbachev has called for Russia's parliament election to be annulled and a new vote held. He said the results do not reflect the will of the people, and authorities must admit to fabrications and rigging.

Opposition leaders and international observers say the poll Sunday was marred by widespread allegations of vote-rigging, ballot-box stuffing and other irregularities

Thousands of police and Interior Ministry troops are patrolling the streets of Moscow in a show of force aimed at avoiding a third day of protests against the alleged election fraud. More than 250 opposition protesters were arrested Tuesday in Moscow, and more than 100 were taken into custody in St. Petersburg.

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.

U.S. Secretary of State Hillary Clinton said Tuesday the elections were neither free nor fair, and the results raised “serious concerns.”

Russia's Foreign Ministry described Clinton's remarks as “unacceptable.” Russian President Dmitry Medvedev characterized the elections as “fair, honest and democratic.”