BANGKOK: Voting was underway in Thailand on Sunday in an election expected to deliver big gains for opposition forces, testing the resolve of a pro-military establishment at the heart of two decades of intermittent turmoil in the coup-prone country.

About 52 million eligible voters are choosing among progressive opposition parties - one with a knack for winning elections - and others allied with royalist generals keen to preserve the status quo after nine years of government led or backed by the army.

Opinion polls indicate the opposition Pheu Thai and Move Forward parties will gain the most seats but with no guarantee either will govern because of parliamentary rules written by the military after its 2014 coup and skewed in its favour.

Wongsak Na Chiengmai was the first person to vote at his polling station in central Bangkok. "I'm already 88. It's not easy," he said, holding on to a crutch. "This is very important for the country."

Elsewhere in the capital, prime ministerial hopefuls for the ruling party and opposition groups cast their votes early on Sunday, including incumbent Prime Minister Prayuth Chan-ocha.

The election again pits Pheu Thai's driving force, the billionaire Shinawatra family, against a nexus of old money, military and conservatives with influence over key institutions that have toppled three of the populist movement's four governments.

The seeds of conflict were sown in 2001 when Thaksin Shinawatra, a brash capitalist upstart, was swept to power on a pro-poor, pro-business platform that energised disenfranchised rural masses and challenged patronage networks, putting him at odds with Thailand's established elite.

Thaksin's detractors in the urban middle class viewed him as a corrupt demagogue who abused his position to build his own power base and further enrich his family. Mass protests broke out in Bangkok during his second term in office.

In 2006 the military toppled Thaksin, who fled into exile. His sister Yingluck's government suffered the same fate eight years later. Now his daughter Paetongtarn Shinawatra, a political neophyte, has taken up the mantle.

DICTATORSHIP TO DEMOCRACY

"May 14 will be a historic day. We will change from a dictatorship to a democratically elected government," Paetongtarn, 36, told cheering crowds on Friday at Pheu Thai's final rally.

"Every time we come to power we are able to bring prosperity to the people. I've entered politics to help the new generation, to support their families."

The populist approach of Pheu Thai and its predecessors has been so successful that rival forces that once derided it as vote-buying now offer strikingly similar policies.

The military-backed Palang Pracharat promises a handout of 30,000 baht ($890) each to 7.5 million farming families, a big increase in allowances for the elderly and infrastructure projects in Thailand's poorest region.

The United Thai Nation of Prime Minister Prayuth, who led the coup against Pheu Thai's last government, has pledged debt relief, cheaper electricity for low-income groups and subsidies for transport and crop harvesting.

Prayuth has campaigned on continuity, hoping to woo conservative middle-class voters tired of street protests and political upheaval.

"We do not want change that will overturn the country. Can you accept that? Do you know what kind of damage it would do?" he asked supporters on Friday.

Some analysts argue the fight for power in Thailand is more than a grudge match between the polarising Shinawatra clan and its influential rivals, with signs of a generational shift and hankering for more progressive government.

Move Forward, led by 42-year-old Harvard alumnus Pita Limjaroenrat, has seen a late surge. It is banking on young people, including 3.3 million eligible first-time voters, to back its plans to dismantle monopolies, weaken the military's political role and amend a strict law against insulting the monarchy that critics say is used to stifle dissent.

"The election is a test of the conservative roots and the future of progressiveness," said Ben Kiatkwankul, partner at government affairs advisory Maverick Consulting Group.

"The issue is bigger than whether people like or dislike Thaksin or Prayuth. Now it's the old system facing off against the liberalist wave."

($1 = 33.85 baht)