Tens of thousands of police will be deployed across Thailand on Sunday for an election seen as a crucial test of the kingdom's fragile democracy, with opposition protesters threatening to lay siege to polling stations.

Experts warn the vote is unlikely to end a long-running cycle of political violence or mollify opponents of Prime Minister Yingluck Shinawatra who fear the polls will only prolong her billionaire family's hold on power.

At least 10 people have been killed and hundreds injured in clashes, grenade attacks and drive-by shootings since mass opposition rallies against her government began three months ago.

"Thailand seems to be in a perpetual state of conflict right now and I don't see any end in sight," said Sunai Phasuk, a senior researcher with New York-based Human Rights Watch.

The elite-backed opposition Democrat Party -- which has not won an elected majority in around two decades -- is boycotting the vote, tilting the odds toward another victory by Yingluck's party but reinforcing questions about whether the new parliament will have enough members to sit.

Protesters, who have occupied key intersections of Bangkok, are demanding Yingluck's elected government step down to make way for an unelected "people's council" that would oversee loosely defined reforms to tackle corruption and alleged vote-buying.

Yingluck's opponents say she is a puppet for her elder brother Thaksin Shinawatra, a controversial former prime minister who was ousted by the military in 2006 and lives in Dubai to avoid a prison term for graft.

The protests were initially triggered by a failed amnesty bill that could have allowed Thaksin to return without going to jail.

'Political limbo' looms

Fifty-three parties are taking part in Sunday's election, hoping to fill the void left by the Democrats, although there has been little sign of campaigning in the capital apart from a few defaced election posters.

Protest leader Suthep Thaugsuban, an opposition heavyweight, has urged supporters to do their utmost to prevent the polls taking place, raising fears of further violence.

His bold threat comes despite a 60-day state of emergency imposed in the capital and surrounding areas that gives authorities extra security powers.

Even if voters manage to cast their ballots, election officials warn that the poll result may not be known for months because of problems caused by the protests.

Disruption by demonstrators to candidate registrations means that if Yingluck wins she will remain in a caretaker role with limited power over government policy until by-elections are held to ensure there are enough MPs to convene parliament.

Thailand faces a "legal and political limbo that never happened before", warned Sunai.

"It's no longer necessary to have tanks on the street to remove one side of the political divide from the scene," he said. "Without a parliament there can be no elected government."

Advance voting in parts of the country, including Bangkok, on January 26 was marred by blockades by opposition protesters who stopped hundreds of thousands of people from casting ballots.

On Sunday 129,000 police will be deployed to protect ballot boxes and guard more than 93,500 polling stations, said deputy national police spokesman Anucha Romyanan.

'Elite clash of interests'

The backdrop to the protests is a years-long political struggle pitting the kingdom's royalist establishment -- backed by the courts and the military -- against Thaksin, a billionaire tycoon-turned-politician.

The dispute comes at a time of national anxiety about the health of 86-year-old King Bhumibol Adulyadej, and about who will be running the country when the revered but ailing monarch's more than six-decade reign comes to an end.

"You have in Thailand an elite clash of interests," said Paul Chambers, director of research at the Institute of Southeast Asian Affairs at Chiang Mai University in northern Thailand.

Thaksin and his allies rely on support among the rural poor to win elections, he said.

"On the other hand the royalists have the military and the judiciary. It's a never-ending struggle," Chambers said.

Pro-Thaksin parties have won every election since 2001, most recently with a landslide victory under Yingluck two years ago.

In addition to the 2006 coup, two pro-Thaksin premiers were forced from office in 2008 by the courts, angering the ousted leader's "Red Shirt" supporters who have vowed to rise up if another elected government is removed by the army or the judiciary.

When the Red Shirts took to the streets in 2010 demanding new elections, more than 90 people died and nearly 1,900 people were wounded in street clashes and a military crackdown under the previous government.