Jakarta, Apr 15 (AFP) Dipping their fingers in halal ink to prevent double voting, Indonesians cast their ballots Wednesday in a bitterly contested presidential election, with the main rival to incumbent Joko Widodo already threatening to challenge the result over voter-fraud claims.

The Muslim-majority nation's biggest-ever polls -- with more than 190 million voters and 245,000 candidates vying for the presidency, parliament and local positions -- is largely a referendum on Widodo's infrastructure-driven bid to rev up Southeast Asia's largest economy.

But, looming in the background, two decades of democratic gains are at risk of being eroded, analysts said, as the military creeps back into civilian life under Widodo, and his trailing rival Prabowo Subianto, a former general, eyes reforms that harken back to the Suharto dictatorship.

If he loses, Subianto's camp has already warned it will challenge the results over voter-list irregularities.

"It's high stakes in this election," said Evan Laksmana, a senior researcher at the Jakarta-based Centre for Strategic and International Studies.

"We simply don't know what (Subianto) would do if he won and we don't know if the institutional constraints in place would contain him."

Voting starts at 7:00 am local time Wednesday (2200 GMT Tuesday) in easternmost Papua and ends at 1:00 pm at the other end of the country in Sumatra.

Ballots will be cast at more than 800,000 polling booths across the volcano-dotted country, from the tip of jungle-clad Sumatra and heavily populated Java island to beach paradise Bali and far-flung Sumbawa.

Voters will punch holes in ballots -- to make clear their candidate choice -- and then dip a finger in Muslim-approved halal ink, a measure to prevent double-voting in a graft-riddled country where ballot buying is rife.

A series of so-called "quick counts" are expected to give a reliable indication of the presidential winner later Wednesday. Official results are not expected until May.

Most polls show the 57-year-old Widodo holding a double-digit lead over Subianto, 67, setting up a repeat of their 2014 contest, which Widodo won despite an unsuccessful court challenge over his narrow victory.

The race has been punctuated by bitter mudslinging between the two camps, religion-driven identity politics and a slew of fake news online that threatens to sway millions of undecided voters.

Widodo campaigned on his ambitious drive to build roads, airports and other infrastructure, including Jakarta's first mass-rapid-transit system.

But his rights record has been criticised owing to an uptick in discriminatory attacks on religious and other minorities, including a small LGBT community, as Islamic hardliners become more vocal in public life.

"(Widodo) has chosen pragmatism over principle on issues of Islamism and pluralism," said Dave McRae, a senior lecturer at the University of Melbourne's Asia Institute.

Widodo, a practising Muslim, blunted criticism that he was anti-Islam by appointing influential cleric Ma'ruf Amin as his running mate.

But victory for Widodo and Amin -- known for his disparaging views towards minorities -- could be the latest knock to Indonesia's reputation for moderate Islam. "There is a longstanding track record of very conservative views," Kevin O'Rourke, an Indonesia-based political risk analyst, said of Amin.

"It's inevitable that will affect policy making." Subianto -- joined by running mate Sandiaga Uno, a 49-year-old wealthy financier -- has run on a fiery nationalist ticket. He courted Islamic hardliners, promised a boost to military and defence spending and, taking a page from US President Donald Trump, vowed to put "Indonesia first" as he pledged to review billions of dollars in Chinese investment.

Subianto's presidential ambitions have long been dogged by strong ties to the Suharto family and a chequered past.

He ordered the abduction of democracy activists as the authoritarian regime collapsed in 1998, and was accused of committing atrocities in East Timor.

Widodo's own cabinet is stuffed with Suharto-era figures, and he raised eyebrows by agreeing to give civilian government jobs to military brass.

But "there is no grand design for Jokowi to bring back military rule", Laksmana said.

Subianto, however, is a military man keen to roll back reforms that ushered in direct presidential elections, analysts said.

That has raised questions about what an upset victory for the retired general could mean for a system that is supported by most Indonesians.

"Democracy itself would be very much at stake," O'Rourke said.

"This is a low probability scenario, but one with very high impact." Many Indonesians just want a peaceful power transition -- regardless of the winner. "I hope there's no hostility," said 53-year-old Untung Sri Rejeki. "No matter who becomes our next president." (AFP)

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