Defense Minister Prabowo Subianto declared victory in Indonesia's presidential election in his third attempt, according to unofficial results on Wednesday.

Former special forces commander Prabowo clinched about 58% of votes according to unofficial "quick counts" by four independent pollsters, more than double that of his nearest opponent, former Jakarta governor Anies Baswedan.

A much slower, preliminary tally by the election commission with 18% of votes counted also pointed to a Prabowo rout of his rivals, putting the 72-year-old political veteran on a trajectory for a decisive single-round win.

"This victory should be the victory for all Indonesians," Prabowo said in a rousing speech at a stadium that drew roars and applause from supporters.

"We will assemble a government consisting of the best sons and daughters of Indonesia."

The contest pitted popular former governors Anies and Ganjar Pranowo against pre-election frontrunner Prabowo, who was feared in the 1990s as a top lieutenant of the late strongman Suharto, who ruled Indonesia as an autocrat for three decades.

Prabowo crucially has the tacit backing of the wildly popular incumbent Joko Widodo, who has bet on his former rival as a continuity candidate to preserve his legacy, bolstered by the inclusion of the president's son Gibran Rakabuming Raka as Prabowo's running mate.

Anies and Ganjar trailed with about 25% and 17% respectively, with 88% to 98% of ballots counted by the four pollsters conducting sample counts, which in previous elections have proven to be accurate.

To win outright and avoid a second-round runoff, the leading candidate needs more than 50% of votes cast and at least 20% of the ballot in half of the country's provinces.

Anies said he would honor and respect the official result, which is expected by March 20 at the latest, and vowed to continue what he called a "movement of change".

"We will wait until all the counts are done ... it is our commitment as fighters for democracy," he told supporters.

The campaign teams of Ganjar and Anies had earlier said they were investigating reports of electoral violations, both calling it "structural, systematic and massive fraud", but provided no evidence.

The world's biggest single-day election saw nearly 259,000 candidates and 18 parties contest 20,600 posts across the archipelago of 17,000 islands. The U.S. State Department said the vote was "a testament to the durability and strength of the Indonesian people's commitment to the democratic process."

A coalition of parties backing Prabowo had about 42% of votes according to the unofficial counts in the legislative contest, while an alliance behind Anies had 27%, indicating a potential Prabowo government could have strong parliamentary backing.

But top billing on Wednesday was always the race to replace Jokowi, as the incumbent is known, whose entrenched influence after a decade in power could prove to have been decisive.

Prabowo has transformed his image as a fiery-tempered nationalist and notorious military hardliner to a cuddly, cat-loving grandfatherly figure, attracting a huge youth following on social media in a country where more than half of the nearly 205 million electorate are under 40.

But central to his appeal has been the president's implied endorsement and popularity that has rubbed off on Prabowo, who some analysts say could help the outgoing Jokowi to retain his political clout and deliver on ambitions to modernize infrastructure and turn Indonesia into an electric vehicle hub.

Arya Fernandes of Indonesia's Center for Strategic and International Studies said the unofficial count indicated the "Jokowi effect" had worked for Prabowo and a second round was unlikely.

"There is a tendency among voters to be drawn to strong leaders," he said. "The effectiveness of the 'political support' from the incumbent has been a contributing factor."

Jokowi, who has reached his term limit, has not explicitly backed a candidate and has endured a storm of criticism and allegations of interference, including in a decision by a court headed by his brother-in-law that changed eligibility rules, which allowed his son to become Prabowo's running mate.

Jokowi's loyalists have rejected allegations of meddling.

Ganjar has campaigned largely on continuing the president's policies as a member of the same party, but crucially lacked Jokowi's endorsement. He remained upbeat on Wednesday, but said his camp would investigate reports of fraud, which he did not detail.

"No struggle is in vain. And of course, everyone is still enthusiastic," he said.

But the margin of Prabowo's lead in the vote counts, preliminary and unofficial, could make it difficult for rivals to launch a viable challenge to the outcome.

"This is an emphatic result that diminishes prospects for legal challenges and will also provide the Widodo-Prabowo alliance with an element of affirmation from the public for decision-making in the months ahead," said political analyst Kevin O'Rourke.