MOSCOW | Sun Dec 4, 2011 3:35pm EST

(Reuters) - Vladimir Putin's ruling party suffered a serious blow in a parliamentary election on Sunday, exit polls showed, as voters signalled growing unease with his domination of Russian politics before a planned return to the presidency next year.

The result, in which the opposition said Putin's United Russia was boosted by fraud, is likely to dent the authority of the man who has ruled for almost 12 years with a mixture of hard-line security policies, political acumen and showmanship.

Two exit polls suggested Putin's party, United Russia, would win 45.5 to 48.5 percent of the votes in the election to the State Duma, compared with 64.3 percent in 2007, and that it could struggle even to hold on to a majority in the chamber.

These elections are unprecedented because they were carried out against the background of a collapse in trust in Putin, [President Dmitry] Medvedev and the ruling party, said Vladimir Ryzhkov, a liberal opposition leader barred from running.

I think that the March [presidential] election will turn into an even bigger political crisis; disappointment, frustration, with even more dirt and disenchantment, and an even bigger protest vote.

Putin made his mark restoring order in a country suffering from a decade of chaos following the collapse of the Soviet Union. He moved quickly to crush a separatist rebellion in the southern Muslim Chechen region, restored Kremlin control over wayward regions and presided over an economic revival.

He has maintained a tough-man image with stunts such as riding a horse bare chested, tracking tigers and flying a fighter plane. But the public appears to have wearied of the antics and his popularity has fallen.

Last month he was booed when he spoke at a sports meeting, and many voters are fed up with widespread corruption and a huge gap between the wealthy and the poor.

Putin and Medvedev, who took up the presidency in 2008 when Putin was forced to step down after serving a maximum two consecutive terms, made a brief appearance at a subdued meeting at United Russia headquarters.

Medvedev said United Russia, which had previously held a two thirds majority, allowing it to change the constitution without opposition support, was prepared to forge alliances on issues.

This is an optimal result which reflects the real situation in the country, Putin, 59, said. Based on this result we can guarantee stable development of our country.

But there was little to cheer for the man who has dominated Russian politics since he came to power in 1999, then to be elected president when Boris Yeltsin resigned months later.

Two decades after the collapse of the Soviet Union, the communists appeared to be the main beneficiaries in opposition ranks, their vote almost doubling to around 20 percent, according to the exit polls.

Russia has a new political reality even if they rewrite everything, said Sergei Obukhov, a communist parliamentary deputy.

Opposition parties complained of election irregularities in parts of the country spanning 9,000 km (5,600 miles), and a Western-financed electoral watchdog and two liberal media outlets said their sites had been shut down by hackers intent on silencing allegations of violations.

Sites belonging to the Ekho Moskvy radio station, online news portal Slon.ru and the watchdog Golos went down at around 8 a.m.

Massive cyber attacks are taking place on the sites of Golos and the map showing violations, Golos said on Twitter.

Medvedev has dismissed talk of electoral fraud.

WORSE IN THE CITIES

Independent political analyst Dmitry Oreshkin said a separate analysis showed that United Russia fell even further in cities -- where it had between 30-35 percent of the votes and the Communist have 20-25 percent.

No one expected such a sharp downfall for United Russia ... This is a bad climate for Putin. He has got used to the fact that he controls everything, but now how can he go into a presidential campaign when United Russia has embittered people against their leader?

Putin is still likely to win the March 4 presidential election and could extend his rule until 2024 if he wins the maximum two more terms.

The result was also a blow for President Dmitry Medvedev, slated under a deal with Putin to take over the prime minister's office after the March election. He led the campaign and his position might now be in question.

Opposition parties say the election was unfair from the start because of authorities' support for United Russia with cash and television air time.

Putin has no serious personal rivals as Russia's leader. He remains the ultimate arbiter between the clans which control the world's biggest energy producer.

But his party has had to fight against opponents who have branded it a collection of swindlers and thieves and combat a growing sense of unease among voters at Putin's grip on power. ($1 = 30.8947 Russian roubles)

(Writing by Timothy Heritage, Editing by Ralph Boulton)