Indonesian Defence Minister Prabowo Subianto was on course to win the archipelago's presidential poll by a wide margin, official tallies showed Friday with more than half of votes counted.

The final result is not expected until late March but early indications all point to the 72-year-old ex-general succeeding popular outgoing leader Joko Widodo.

With more than half the ballots counted, Prabowo had a commanding 57 percent of votes, more than double his nearest rival and enough for a first-round majority, the election commission's website showed.

Former Jakarta governor Anies Baswedan was on 24.98 percent on Friday morning and former Central Java governor Ganjar Pranowo had 18.02 percent.

"Thank God, we must be grateful and continue to monitor the KPU's official results," Prabowo wrote on Instagram late Thursday, referring to the general election commission.

The fiery populist on Wednesday claimed a "victory for all Indonesians" alongside his running mate -- the current president's eldest son Gibran Rakabuming Raka -- based on preliminary results by government-approved pollsters.

The early sample counts -- previously shown to be reliable -- showed they were set for a first-round majority. Gibran, 36, would become Indonesia's youngest-ever vice president.

But both of his rivals said they would wait for the official result and had not conceded.

Prabowo needs more than 50 percent of the overall vote and at least a fifth of ballots cast in more than half the country's 38 provinces to officially secure the presidency.

Analysts said his win was almost assured.

Jokowi, as the incumbent leader is popularly known, told reporters Thursday he had met with Prabowo the previous evening to offer his congratulations.

He has been accused of backing his former rival and defence chief's campaign in a bid to install a political dynasty, via his son, before leaving office.

In his Instagram post, Prabowo said Australian Prime Minister Anthony Albanese had called to congratulate him, as well as the leaders of Singapore, Malaysia and Sri Lanka.

Mark Rutte, leader of Indonesia's former colonial ruler the Netherlands, tweeted late Thursday he had congratulated the president-in-waiting because of the "projected outcome".

The United States was more cautious, only congratulating the Indonesian people on the election's "robust turnout" in a statement that did not mention Prabowo.

Calls for justice 

Prabowo's election rivals have said they would investigate if there was any fraud in the vote, with Ganjar's team saying it had found "systematic" fraud, without providing evidence.

Anies on Friday visited Al-Azhar Great Mosque in the capital Jakarta, telling reporters "the deficiencies are various" in the vote count, also without providing evidence.

Reports of irregularities "can be seen on social media", he said, calling on people to report incidents to his legal team.

Holding one of the world's biggest single-day elections takes its toll on poll workers who count votes by hand, sometimes through the night, and deaths from overwork-related conditions and accidents are common.

At least 14 poll workers died from Tuesday to Thursday, according to local media reports Friday, citing local government, police and election officials.

While Prabowo supporters reacted with jubilation outside his home and at a packed arena in Jakarta after polls closed, activists whose children were shot dead or disappeared by military forces in the 1990s protested his win.

Non-governmental organisations and his former bosses accuse Prabowo of ordering the abduction of democracy activists towards the end of dictator Suharto's three-decade rule. More than a dozen have never been found.

Prabowo was discharged but has denied responsibility and was never charged.

Outside the presidential palace, more than a hundred protesters gathered late Thursday, holding up yellow cards, blowing whistles and unfurling a banner that read "save democracy".

One of them was Paian Siahaan, whose son Ucok disappeared in the last months of Suharto's rule, when Prabowo was a top commander.

Ucok was 22 when he went to a protest and never came back.

"This is beyond our prediction after following the campaigns and debates. We didn't anticipate that he would win by such a wide margin," said Paian, 77.

"So we are consoling each other."