Moscow (CNN) -- While Russian police arrested 170 opposition protesters in two cities, voters in the country's parliamentary election appeared to eat away at Prime Minister Vladimir Putin's support in Sunday's elections, slimming down his party's parliamentary majority.
With more than 61% of the total vote counted, Putin's United Russia party is far and away the winner of Sunday's elections with 50%, followed by 19.5% for the runner-up Communist Party, according to the All-Russia Center for Public Opinion. The Fair Russia party has 12.9% and the Liberal Democratic party has 11.9%.
But the poll numbers add up to a significant loss. If they are an accurate reflection of the election results, United Russia stands to lose many of the 300 seats it currently holds in the 450-seat Duma -- Russia's parliament.
President Dmitry Medvedev, who headed the United Russia ticket, said the party made a "decent showing," and that "the result of these parliament elections reflects people's attitudes"
"United Russia remains the leader and the largest political force elected to the parliament," Medvedev said in remarks at his party's headquarters. "The party has proven it has a moral right to continue the chosen course."
Putin, who spoke after Medvedev, thanked those who voted for his party "despite the difficulties, despite the economic crisis."
"Based on this result, we will manage to ensure the stable development of our state," Putin said. "I would like to thank everybody who facilitated this result."
Analysts had anticipated Putin's party would win less support than four years ago -- but would maintain a majority. With three parties not receiving enough votes to take a seat in parliament, as the opinion poll indicates, then United Russia could still hold on to majority.
Around 100 opposition protesters were arrested in Moscow according to official news agency RIA Novosti, which sourced police. Authorities detained 70 more in St. Petersburg. Police had warned protesters earlier in the day not to hold "unsanctioned rallies" in Moscow, the Interfax news agency reported.
Opposition websites, radio stations and an election monitoring group claimed they had come under online attack.
The Golos election watchdog organization said callers reported about 1,000 elections violations on a telephone hotline while its website was under cyber attack. Russia's Interfax news agency reported that several other radio and newspaper websites had reported attacks.
The allegations came as voters cast their ballots in polls for the State Duma, the lower house of Russia's parliament.
"It's a very important test for the ruling party," Dmitry Babich, a political analyst with RIA Novosti, told CNN.
Prime Minister Vladimir Putin, recently tapped by his United Russia Party to be its presidential candidate next year, has accused the West of trying to influence the elections.
In campaigning ahead of the vote, opponents accused the ruling party of corruption and nepotism, RIA Novosti reported.
Putin said last week that his party had earned the support of "every thoughtful, objective, serious person who wants a better lot for himself, for his children and for Russia," the news agency said.
Russia's Interior Ministry opened three criminal cases and reported hundreds of other "electoral breaches," RIA Novosti said, citing the ministry's press office.
Moscow police said they detained about 12 people who were distributing political leaflets -- a practice banned on election day.
Golos said there was increasing pressure at the local level to block observers from accessing polls.
"It is clear that these actions are taken by authorities to undermine the achievement of our long-term goal -- to make the elections in Russia free and fair by impartial and independent monitoring," the organization said in a statement.
Maria Lipman, an analyst at the Carnegie Moscow Center, told CNN Sunday that elections had become "increasingly farcical during Putin's leadership."
"The last time we had an election at the federal level without a pre-ordained result was '99," she said. "Since then all elections have pre-ordained results and were to maintain the political monopoly of the ruling elite."
In a taped interview with Russia's national TV networks in September, President Medvedev criticized allegations that Russia's elections had a predetermined outcome.
"I consider such statements absolutely irresponsible, deceitful, and even provocative," he said.
In September Medvedev called on the United Russia party to endorse Putin for president in 2012. Putin in turn suggested that Medvedev should take over the role of prime minister if the party wins elections, in what would be a straight swap of their roles.
The announcement ended more than two years of speculation about whether Putin or Medvedev -- Putin's hand-picked successor -- would seek to run for a second term.
Putin stepped down as president in 2008 because the Russian constitution at that time limited the office to two consecutive four-year terms.
Under amendments to the constitution that came into force on December 31, 2008, the presidential term was extended to six years.
This means that if Putin is elected in March 2012 for six years, he would be eligible to run for another six-year term after that, potentially keeping him in charge until 2024.
CNN's Alla Eshchenko, Natalie Allen and Maxim Tkachenko contributed to this report