The commission chairman spoke to The Associated Press on condition of anonymity for fear of losing his job. He also said he could be punished for disobeying orders to report any contact with foreign observers or journalists to the FSB, the successor to the Soviet-era KGB.

His account closely matches reports by independent observers of rampant vote-rigging during Sunday's election, in which United Russia maintained its majority in parliament. Amateur videos posted on the Internet also appeared to show falsified ballots spilling out of boxes at polling stations.

Officially, United Russia got roughly 50 percent of the vote, a significant drop from the 64 percent the party won in the last election. But the reports of fraud indicate it may have lost even more support than those results suggest. Central Election Commission officials said they have received no reports of serious violations but would investigate any formal complaints.

This election was emerging as a watershed moment in a country where people have long seemed inured to vote manipulation, both before and after the fall of the Soviet Union. The fraud allegations have set off protests in the street and stirred broader public indignation, suggesting that the political system Putin built to solidify his control has begun to crack just three months ahead of a vote on his return to the presidency. The lackluster opposition has suddenly been energized.

Anger over the election drew more than 5,000 people Monday night, in one of the biggest anti-Putin protests in years. Police detained about 300 protesters to prevent them from marching to the Central Elections Commission near the Kremlin. New protests on Tuesday night were thwarted by police, who were out in force after having been taken by surprise the night before.

Amateur videos claiming to show the vote being rigged have spread via social media networks, including one in which the chairman of an election commission is filling out a stack of ballots. The clip attracted so much attention that city election officials were forced to acknowledge that the chairman had been caught falsifying the vote and could face charges.

The commission chairman who spoke to the AP said that representatives of Russia's four main parties got together before the election to negotiate how many votes each would get in district precincts. United Russia initially wanted 68 to 70 percent, but conceded that was too high and settled for around 65.

On voting day, the chairman said, election workers quietly slipped ballots into the boxes, as many as 50 at a time, being careful to keep the papers from rustling and attracting the attention of observers.

He said workers were trained on how to stuff ballots, each a thin sheet roughly the size of standard letter paper. He demonstrated how a stack of up to 30 or even 50 ballots could be folded in half, hidden inside a jacket and slipped into the ballot box without making any noise.

The chairman said there was a limit to the amount of ballot stuffing his commission could do. So district election officials took a few hundred of the precincts ballot, filled them out for United Russia and gave them to migrant men not on the precinct's rolls. Fake voter lists were substituted for the real ones.

During a tour of the polling station, the commission chairman pointed to a spot along the far wall where he had put chairs for observers. He said one observer was particularly zealous, never leaving the room during the 12-hour voting period, even to use the toilet.

The chairman said he got the police to evict the observer 10 minutes before the polls closed, too late for a replacement to be sent. Election monitoring groups and political parties have complained that their observers were barred from many polling places.

When the votes were counted, United Russia got only about 50 percent, even with all the extra ballots; the chairman said its real support had been about 25 percent. Turnout also was low, another setback for the Putin camp.

But when the chairman reported the 50 percent result to the district election commission, he said, he was told to make it 65 percent in the official report, which needed to be signed by all 15 members of the commission. Turnout was also to be inflated.

Most of the commission members willingly went along with the change, he said, and the one holdout was appeased when a few dozen votes were taken from smaller parties and given to the Communists.

The director of Golos, an independent election watchdog, said chairmen of election commissions at polling stations are at the center of efforts to rig the vote and routinely come under heavy pressure.

``Most of the violations we see happen at the local level,'' Liliya Shibanova said.

Golos says many violations involved busing people with absentee ballots to multiple polling stations so they can vote, a system called ``cruise'' or ``carousel'' voting.

Putin, who served as president from 2000 to 2008 and then moved into the prime minister's office because of presidential term limits, is hoping to return to the presidency after the March election. He had been counting on a strong show of popular support for United Russia in the parliamentary election to add legitimacy to his campaign.

He has appeared shaken by the election results and by the overall lack of enthusiasm over his decision to reclaim the presidency from Dmitry Medvedev.

Many Russians are growing weary of his leadership, and of the pervasive corruption and great social inequality it has fostered.

Still, there is little doubt that Putin will win the presidential election. He remains more popular than his party and will likely face only tepid opposition, given his control over who is allowed to run.

Putin seems to realize that he needs to respond to the discontent, but gave no sign Tuesday that he knows how.

``As for the question of what exactly is worrying people and why they don't vote for United Russia but vote for other parties, of course we need to think about this,'' he said. ``We need to analyze these problems and formulate further suggestions on solving them.''