Former Soviet leader Mikhail Gorbachev has called for Russia's parliamentary 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.

The 80-year-old Mr. Gorbachev said Wednesday that ignoring public opinion discredits authorities and destabilizes the situation.

Opposition leaders and international observers say Sunday's elections were marred by widespread allegations of vote-rigging, ballot-box stuffing and other irregularities.

Fifty-thousand police officers backed by helicopters patrolled the streets of Moscow Wednesday in a show of force aimed at avoiding a third day of protests against the alleged election fraud. With a social networking site promising nightly protests, police late Wednesday arrested 50 demonstrators in Moscow and 100 in St. Petersburg. Security officials said 550 people had been arrested in Moscow protests the night before.

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.

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.”