By Andrew Osborn and Maria Tsvetkova
MOSCOW (Reuters) - Vladimir Putin cemented his supremacy over Russia's political system when his ruling United Russia party took three quarters of the seats in parliament in a weekend election, paving the way for him to run for a fourth term as president.
Opposition activists and European observers questioned how free and fair the vote had been, however, although there were no immediate signs the result might spark street protests of the kind which erupted after the last such election in 2011.
With most votes counted, United Russia, founded by Putin almost 16 years ago after he first became president, was on track on Monday to win 76 percent of the seats in Russia's Duma, the lower house of parliament, up from just over half in 2011.
That would be its biggest ever majority. Putin's spokesman called it "an impressive vote of confidence" in the Russian leader and dismissed critics who noted a sharp fall in turnout.
Around 4 million fewer Russians voted for United Russia compared to 2011, data from the Central Election Commission showed, while overall turnout fell to 48 percent from 60 percent, exposing growing apathy about a political system and elite which critics say tolerates no genuine opposition.
Liberal opposition parties failed to win a single seat. Dmitry Gudkov, the only liberal opposition politician to hold a seat before, said he had been beaten by a United Russia candidate whose chances he said had been lifted by poor turnout.
"The question now is...how to live with a one-party parliament," Gudkov said.
European election monitors said the vote was marred by numerous procedural irregularities and restrictions on basic rights. Russian officials said there was no evidence of widespread fraud.
Near complete results showed especially low turnout in Moscow and St. Petersburg, where protests against Putin erupted after the last parliamentary election in 2011. Putin and his allies have since tightened protest laws and made life difficult for civil society groups which take money from outside Russia, branding them "foreign agents."
VOTE FOR STABILITY
Presiding over a government meeting at the Kremlin on Monday, Putin hailed the election result, saying it showed voters still trusted the country's leadership despite an economic slowdown made worse by Western sanctions over Ukraine.
"The result is a good one for United Russia. At a time of difficulties, considerable uncertainty and risks, people naturally choose stability and trust the leading political force," said Putin.
"The results ... are also citizens' reaction to external attempts to pressure Russia, to threats, to sanctions, and to foreign attempts to stir up the situation in our country."
Putin's aides are likely to use the result as a springboard for his own re-election campaign in 2018, though he has not yet confirmed whether he will seek another term.
United Russia won 343 seats of the total of 450 in the Duma, the Central Election Commission said, after 93 percent of ballots had been counted.
That is up from 238 seats in the last parliamentary election and enough to allow United Russia to unilaterally change the constitution, though Putin can run again for the presidency under the existing one because he was prime minister between his second and third terms.
Other parties trailed far behind.
According to the near complete official vote count, the Communists were on track to come second with 42 seats, the populist LDPR party third with 41, and the left-of-center Just Russia party fourth with 21 seats. All three tend to vote with United Russia on crunch issues and avoid direct criticism.
Among voting irregularities witnessed by Reuters were several people voting twice in one polling station in the Mordovia region of central Russia. Official results in another area showed a turnout double that recorded on the spot.
Ilkka Kanerva, special coordinator for the elections from the Organization for Security and Cooperation in Europe, said the OSCE had noted some improvements, including greater transparency when it came to administration.
But he said the overall picture was beset by problems. "Legal restrictions on basic rights continue to be a problem. If Russia is to live up to its democratic commitments, greater space is needed for debate and civic engagement," he said.
Speaking to United Russia campaign staff a few minutes after polling stations closed on Sunday night, Putin said turnout, which was 12 percent lower than the 2011 vote, was "not as high as we saw in previous election campaigns, but it is high."
His spokesman Dmitry Peskov said the "overwhelming majority" of voters had come out for Putin in "an impressive vote of confidence". It would be wrong to call the turnout low, he said, adding that it was higher than in most European countries.
Leonid Volkov, a prominent opposition activist, said he thought the scale of fraud was comparable to 2011 and that more than a third of the some 28 million votes which United Russia won had been "made up."
The return of an old voting system, under which half, rather than all, deputies were drawn from party lists with the other half decided by votes for individuals, boosted United Russia's seats. It says the vote was clean. Near final results gave it 140 seats via the lists and 203 from the constituency system.
Members of the upper house of parliament, the Federation Council, are chosen by politicians in Russia's more than 80 regions who, in most cases, are loyal to United Russia.
The party benefits from its association with 63-year-old Putin, who, after 17 years in power as either president or prime minister, consistently wins an approval rating of around 80 percent in opinion polls.
Most voters see no viable alternative to Putin and fear a return to the chaos and instability of the 1990s, the period immediately after the collapse of the Soviet Union.
Many voters are also persuaded by the Kremlin narrative, frequently repeated on state TV, of the West using sanctions to try to wreck the economy in revenge for Moscow's seizure of Crimea, the Ukrainian region it annexed in 2014.
Putin has said it is too early to say if he will run in 2018. If he did and won, he would be in power until 2024, longer than Soviet leader Leonid Brezhnev, the longest-serving Soviet leader aside from Joseph Stalin.
(Additional reporting by Dmitry Solovyov and Polina Devitt in Moscow, Jack Stubbs, Olga Sichkar and Svetlana Burmistrova in Ufa, Andrei Kuzmin in Velikiye Luki, Gleb Stolyarov, Alex Winning and Vladimir Soldatkin in Saransk, Anton Zverev in the Tula Region, Anastasia Teterevleva in the Moscow Region, Maria Tsvetkova, Kira Zavalyova, Denis Pinchuk and Oksana Kobzeva; Writing by Christian Lowe and Andrew Osborn; Editing by Philippa Fletcher)