Tens of thousands of Russians have rallied on a Moscow street to protest alleged fraud in the December 4 parliamentary election won by the ruling party of Prime Minister Vladimir Putin.

Organizers say more than 100,000 people joined the rally on Sakharov Avenue in below-freezing temperatures Saturday, exceeding the size of a similar protest earlier this month. Russian police estimated the turnout at about 28,000 people.

Many protesters wore white ribbons and held white balloons – symbols of a new movement of liberals, nationalists and other groups calling for free elections in Russia and an end to Mr. Putin's 12 years in power.

Former Soviet president Mikhail Gorbachev also urged Mr. Putin to heed the demands of protesters and quit politics. In a Saturday interview with Moscow's Echo radio, Mr. Gorbachev said Mr. Putin's three terms in power were enough.

The activists say Mr. Putin's United Russia party won a narrow majority in this month's parliamentary elections illegally, through vote fraud. They want to throw out the results and hold a new vote. They also say measures are needed to prevent more fraud in Russia's presidential election in March, when Mr. Putin hopes to return to the post he held from 2000 to 2008.

Several opposition figures addressed the protest rally, including popular blogger Alexei Navalny.

He urged demonstrators to continue to lobby for change in the Kremlin.

“I'm happy that we not only made our demands, but we are able to demand further on. And we will insist on it, and we will make them give back what is rightfully ours.”

Ahead of Saturday's rally, a human rights panel that advises the Kremlin sided with protesters and said the December 4 elections were so tainted by fraud that they should be re-run.

The group also called for the resignation of Russia's election commission chief.

The Moscow demonstrations and smaller anti-Putin rallies in other cities are the largest show of discontent in Russia since the 1990s. Russian President Dmitry Medvedev has responded by promising reforms to allow more competition in elections beyond 2012. He is stepping down in March to make way for Mr. Putin's candidacy.

Mr. Putin has accused the United States of fomenting the anti-government demonstrations. He has derided the protesters, saying their white ribbons resemble condoms. Some participants in Saturday's rally in Moscow mocked the prime minister by holding portraits of him with a condom wrapped around his head and waving condoms blown up like balloons.