The poll, conducted by the state-run All-Russian Public Opinion Research Center, showed United Russia would win the election to the State Duma, Russia's lower house, with 48.5 percent of the votes cast, compared with 64.3 percent in 2007.
The Communist Party emerged in second place, with 19.8 percent of the votes.
Despite the sharp drop in support for United Russia, a party leader -- Boris Gryzlov -- told a news conference in Moscow that "this result that we have achieved is very significant."
The vote has been marred by harassment of election observers and allegations of electoral violations
The December 4 vote pitted seven parties against each other. It is seen as a key test for Putin three months before his all-but-certain return to the Kremlin in the March presidential election.
Putin’s approval rating -- although in gradual decline -- remains at around 60 percent. His promise to rule with a strong hand and maintain stability in Russia remains attractive among some voters who loyally back United Russia, despite its precipitous fall in popularity.
"I’m going to vote for United Russia so that everything remains stable," Sergei, an 80-year-old pensioner, told RFE/RL on his way to vote at a central Moscow polling station. "I've always voted for them. Why? Because I think it will give us stability."
PHOTO GALLERY: Russians Vote, Protest...
But many Russians think the vote will be rigged and that the government is allocating tracts of budget funds in a campaign to keep United Russia in power.
In Vladivostok, The Associated Press reported that voters complained to police that United Russia was offering free food in exchange for promises to vote for the party.
The Communist Party and the liberal Yabloko party complained of extensive election violations aimed at boosting United Russia's vote count, including party observers being hindered in their work.
The deputy chairman of the Central Election Commission, Leonid Ivlev, said that all complaints will be investigated.
"In any case, every [complaint] is being examined, with the help of law-enforcement authorities, the prosecutor's office, and the Investigative Committee," he said.
Websites for numerous news outlets -- including the liberal, independent Ekho Moskvy radio station -- were hit on December 4 by apparent denial-of-service attacks and were inaccessible to the public.
"The attack on the site on the day of the elections is clearly linked with an attempt to hinder the publication of info about violations," Aleksei Venediktov, Ekho Moskvy's chief editor, said in a Twitter message.
Vitaly Kamyshev, a correspondent for RFE/RL's Russian Service, was refused admittance to the Central Election Commission in Moscow, despite having valid accreditation. No explanation was given.
A week ago at a United Russia congress, Putin accused unspecified Western-financed organizations of trying to foment instability in Russia.
Meanwhile, reports said some 100 people taking part in unsanctioned protests in Moscow and 70 activists in St. Petersburg had been detained.
More than a dozen activists from the opposition Left Front movement, including its leader Sergei Udaltsov, were arrested by police on Red Square after unraveling banners decrying the elections as "illegitimate" because some parties were not registered.
WATCH: Left Front rally in Moscow and arrests (in Russian)
The Other Russia opposition party said 24 people, including leader Eduard Limonov, had been detained at a separate unsanctioned rally in the central Triumfalnaya Ploshchad Square.
Police said they also detained 30 people taking part in a St. Petersburg protest against election fraud.
'Many Will Be Upset'
At a polling station in Moscow, opposition figure and former Prime Minister Mikhail Kasyanov spoke to reporters of disillusionment.
"Many will be disillusioned [after the vote]. Many will be upset," Kasyanov said. "Their expectations will have been futile. And now a different life is starting, because March 4 [presidential election] will follow on automatically. Today could have been a point for the start of slow change of the situation in the country, but this will not happen. There won't be any slow change. So people will [now] start to reevaluate the situation in our country."
Another opposition figure, Boris Nemtsov, said he came to the polling station not to vote but to "protest against a farce, against manipulation, against fraud, against the stealing of votes from people."
"[The authorities] convulsively, hysterically, and nastily grab on to power and people see that. Not only the opposition, but ordinary people, too, see this on the Internet, and also everywhere else," Nemtsov said. "So I do not rule out that mass protests will take over the country today as well as in March ."
Many voters have tired of what they see as the authoritarian instincts of the government and the threat of stagnation that hovers over Russia as Putin runs for a third presidential term, 12 years after he first became acting president because of the surprise resignation of President Boris Yeltsin.
Olga, a 36-year old employee at privately owned company in Moscow, said she refused to participate in the last State Duma elections. But she told RFE/RL that her disgust at the party of power has forced her out of her political apathy.
"I'm probably going to vote for Yabloko," she said. "I know they are unlikely to make it [pass the barrier of 7 percent to gain Duma representation]. But I'm doing it to vote against United Russia."
United Russia held 315 of the 450 seats in the last convocation of the State Duma. Public opinion surveys had suggested that United Russia was set to lose its "constitutional majority" as a result of the December 4 vote -- and with it the right to change the constitution and impeach the president.
Research released November 28 by the state-run All-Russian Center for the Study of Public Opinion (VTsIOM) suggested that Vladimir Zhirinovsky's ultranationalist Liberal Democratic Party (LDPR), Gennady Zyuganov's Communist Party, and Sergei Mironov's center-left A Just Russia would win seats.
Three others -- Yabloko, a liberal party whose list is led by economist Grigory Yavlinsky, the center-right party Right Cause, and the nationalist Patriots of Russia -- were expected to struggle to garner enough votes.
'Breach Of Protocol'
On the eve of elections, on December 2, Golos, Russia's only independent election monitor, was fined 30,000 rubles -- about $1,000 -- for what was described by authorities as a breach of election protocol.
Golos, which is partially financed by the European Commission, has recorded more than 5,000 election violations during the campaign ahead of today's voting.
Golos employees say they think they are being targeted by authorities in a campaign to discredit their election monitoring activities.
Late on December 2, the NTV television station aired a report on Golos's foreign funding that its alleged goals are "purely political." NTV is owned by the state-controlled natural-gas monopoly Gazprom.
On December 3, the monitoring group's chief, Lilya Shibanova, was detained at a Moscow airport for 12 hours and her laptop was confiscated.