Vote observers and bloggers said on Saturday they were under attack as Russia prepared for an election likely to slash the ruling party's majority ahead of Vladimir Putin's return as president next year.

The parliamentary elections were widely expected to mark a decline in United Russia's popularity, weeks after the announcement of a controversial job swap between President Dmitry Medvedev and Putin, currently prime minister.

Liliya Shibanova, the head of Russian election monitoring group Golos, said she was held for 12 hours Saturday by customs officials after she landed at Sheremetyevo airport in Moscow.

Officials took her laptop on the pretext it had illegal software, she said.

"This is a provocation directed personally against me," she said, alleging that the authorities were trying to prevent her from travelling to the European parliament next week.

Shibanova said the unprecedented pressure on the 11-year-old organisation built because of its "map of violations" website, where people can upload information, including photo and video evidence of election fraud in Russia.

The map, available at kartanarusheniy.ru, listed a total of 8,334 election violations.

"It is an attempt to paralyse the organisation's work altogether," head of the group's analytical department Alexander Kynev said, adding that many of Golos's 3,000 election observers across Russia are under immense pressure.

Voting in the world's largest country will begin at 2000 GMT Saturday in the Russian Far East regions and end 21 hours later when polling stations close at 1700 GMT Sunday in Kaliningrad, an exclave wedged between Poland and Lithuania.

United Russia currently has 315 seats in the 450-member Duma.

Russian bloggers complained as their most popular website Livejournal was down for the third consecutive day, with some alleging a cyberattack by the authorities to prevent people from discussing Sunday's vote.

"Yes, they are still attacking. They must have a mountain of money," head of Livejournal Ilya Dronov wrote on his Twitter blog as people inundated him with complaints that they were not able to access their accounts.

"One could predict that they would down Livejournal before the elections," another Livejournal employee and popular blogger Rustem Adagamov wrote earlier this week. "And so they have, unfortunately."

Police were on high alert ahead of the polls and expected protests, with Moscow parking lots cleared out around polling stations and over 50,000 officers mobilised to ensure order through the weekend.

Medvedev heads the party list for United Russia, and opinion polls have shown that while it is still almost certain to retain its parliamentary majority, its support may be eroding after years of dominance.

In a message to the Russian people Friday, Medvedev called elections "one of the highest manifestations of democracy."

But 46 percent of Russians expect the vote to be rigged, according to a Levada opinion poll conducted in November.

Opposition parties have also reported harassment of their observers, with the Communist party reporting Thursday that someone broke into its office in the southern city of Krasnodar and broke 150 video cameras purchased for election monitoring.

Opposition movement The Other Russia said masked police raided four Moscow apartments Friday, detaining nine campaigners. "Most had their laptops and cellphones confiscated," group's spokesman Alexander Averin told AFP, adding that one activist ended up with a broken nose after the raid.

The Other Russia and Russia's nationalists plan separate protests in the capital Sunday while around 15,000 pro-Kremlin youth movement Nashi (Ours) have promised a rally of their own to "neutralise" troublemakers.

A legally required campaign blackout went into effect at midnight Friday.

Up to three relatively tame opposition parties are expected to win seats in the next parliament -- the Communist Party, the ultra-nationalist Liberal Democrats and the populist A Just Russia.