MOSCOW - Tens of thousands of Russians staged a peaceful massive protest here Saturday against alleged fraud in last Sunday's parliamentary elections.

As the biggest demonstration of the nationwide wave of protests in Russia, the rally lasted four hours in the afternoon at Bolotnaya Square, across the Moskva river from the Kremlin.

The city's authorities originally gave permission for a 30,000- people rally in the much smaller downtown Revolution Square, but later changed the venue.

People attend a rally in Moscow, December 10, 2011. [Photo/Agencies]

Rally organizers claimed that about 40,000 people had gathered at the square and 10,000 others joined later by marching from the Revolution Square. But according to the Interior Ministry, there were some 25,000 protesters in Moscow.

One hour before the rally began, many protesters already started to pass police security checks and entered the square, holding flags and banners reading "Call for fair elections" and "We demand vote recount."

Some of the banners targeted the country's top leaders, urging them to step down.

"I just feel sick of the regime. But I think the opposition is too motley to be able to change the current situation," Alexander Petayev, a computer programmer, told Xinhua at the Bolotnaya Square.

Another protester, Nisso Mayer, said she came to the rally because she does not trust the election results.

"I know no one voted for the United Russia, so I don't understand how that party won the majority. I'm sure they cheated," said Mayer, a teacher.

A retired military officer, who called himself Victor, said he came to "defend" this country.

"I defended Russia all my career from external threats and now I think I must defend it from inside," he said.

Leaders of opposition parties also attended the rally and chanted slogans together with the crowd.

"We have the right to demand new elections. We have the right to demand that law enforcement agencies open criminal investigations of the thousands of thieves sitting in the election commissions," Sergei Mitrokhin, leader of the liberal Yabloko party which failed to gain enough votes for Duma seats, told local media.

Most protesters admitted they just want the opposition's voice to be heard, not expecting another revolution in the country.

"I hope the authorities can know how strong the (opposition) voice is. I want to see gradual changes, not another revolution," on-line advertising practitioner Tatyana told Xinhua.

Two girls with golden sun-flowers in hands said they carried the flowers to demonstrate a calm way of expressing their anger.

Some other protesters tied white ribbons on their arms, which was suggested by rally organizers to show this was a calm demonstration and they do not want to clash with police.

A number of policemen and vehicles were seen at the rally site, but most police were patrolling the square without carrying fire arms.

At 18:00 local time, people began to leave Bolotnaya Square, with a few still planning to remain there, said a police officer, adding that the situation was calm and no serious incidents have been registered during the four-hour demonstration.

There was so far no comments from Kremlin or the ruling party on Saturday. But Andrei Isayev, a senior United Russia official, said the party would take account of the demands voiced by the protesters.

"There is no doubt that people have a right to protest against the result of the vote or against the way it was handled. I assure you we will hear this rally," he was quoted by RIA Novosti news agency as saying.

Local media said it was the largest public protest in Russia in almost a decade.  

Big protests were also held Saturday in some 20 other cities of Russia, from the Far Eastern city of Vladivostok to the westernmost exclave of Kaliningrad, according to local media.

Massive opposition protests against the alleged vote fraud in favor of the ruling party Untied Russia have taken place in Moscow and St. Petersburg following Sunday's elections, in which the United Russia party gained 238 out of 450 seats in the new State Duma.

Russian Prime Minister Vladimir Putin and President Dmitry Medvedev both promised to investigate alleged vote violations and said citizens have the right to stage lawful protests.

However, Putin on Thursday accused the United States of meddling in Russian affairs by encouraging the opposition protests.