(AFP) - A top Indonesian court was set to rule Thursday on a defeated presidential election challenger's claims that the 2019 race was rigged against him -- allegations that sparked deadly rioting in Jakarta.

Tens of thousands of military and police were deployed in the capital of the world's third-biggest democracy amid fears of more unrest after Thursday's decision, as protesters gathered outside the Constitutional Court.

The court is weighing ex-general Prabowo Subianto's claims that the April poll was plagued by "systematic, structured and massive" electoral fraud that cost him victory.

His lawyers have sought to overturn the official results that declared incumbent Joko Widodo the winner with 55.5 percent of votes, against Subianto's 44.5 percent.

But Subianto's legal team argued that Widodo mobilised the power of the state to win the vote and broke campaign finance rules, and that up to 30 million votes were "stolen".

Denny Indrayana, a lawyer for Subianto, said he was "optimistic" about winning the case.

"They won't reject it," he told AFP, referring to the court.

But election officials have discounted Subianto's cheating claims, and many legal analysts tipped the lawsuit to fail due to weak evidence.

Subianto, who lost a similar court battle in 2014 when Widodo defeated him, has little recourse if the court rules against him again.

"If the claim is rejected then it's game over" for Subianto, said constitutional law expert Refly Harun.

- Heavy security -

However, an unexpected victory could mean holding another election in certain regions or even disqualifying Widodo, he added.

The court hearing began Thursday afternoon as some 47,000 police and army personnel were deployed around the court building and other parts of central Jakarta.

Last month, peaceful protests against the official result by Subianto supporters erupted into two nights of street battles between police and rioters, leaving nine people dead and hundreds injured in the capital's worst violence in years.

Indonesian police have been in the spotlight after videos surfaced that appeared to show officers beating protesters.

There are also questions about how the demonstrators -- including a 15-year-old high school student -- died.

This week, Amnesty International called for an independent probe into what it called "grave human rights violations" by police against protesters -- including beatings and "torture" -- and allegations that they were behind the killings of demonstrators.

Some of the dead were reported to have gunshot wounds. Police have insisted they did not shoot live rounds, but instead used rubber bullets, water cannons and tear gas to push back the crowds.

Meanwhile, several Subianto allies have recently been arrested, including former army general Kivlan Zen over his alleged links to the Jakarta riots.

Police have also aired video from several arrested suspects who claimed that Zen masterminded a failed plot to sow chaos by killing four senior government officials, including its chief security minister and the president's top intelligence adviser.

© Agence France-Presse