JAKARTA (Reuters) - Indonesia’s opposition said on Monday it had uncovered data irregularities affecting millions of people in the election rolls and vowed to take legal action, or use “people power”, if its complaints were not resolved.

FILE PHOTO: An election official records votes in Tangerang, west of Jakarta, Indonesia, June 27, 2018. REUTERS/Willy Kurniawan

The world’s third-biggest democracy holds elections on April 17, with the incumbent, President Joko Widodo, now enjoying a double-digit lead in most opinion polls over his challenger, retired general Prabowo Subianto.

But the opposition is dissatisfied with the response by the election commission (KPU) after it found anomalies, such as errors in dates of birth, or duplicate identity card numbers, affecting 17.5 million people, or about 9 percent of the 192 million voters, an opposition official said.

“We will continue to demand our right, as one of the contestants in this election, that this be resolved as quickly as possible,” Hashim Djojohadikusumo, media director for the opposition campaign, told a news conference.

If not settled, such issues could raise questions over the legitimacy of the vote, he said, citing a chaotic recent election in neighbouring Thailand that provoked criticism of how authorities ran the poll and accusations of manipulated results.

There was a “clear possibility” the opposition could go to the constitutional court unless it obtained satisfactory answers, he said, adding that if fraud was proven then “people power” street protests could also be a legitimate response.

Viryan Azis, a commissioner at the KPU, told Reuters the election commission had “responded positively” to issues raised by the opposition, had clarified the data with other agencies and conducted sampling. The opposition would receive a written response as well as evidence from the field, he added.

“The KPU will work wholeheartedly with all input from the public, especially from election participants,” he added. “We don’t need to be threatened.”

Another KPU commissioner, Wahyu Setiawan, told the Republika news portal that all parties should use legal channels if they felt there was fraud in the election.

Ma’ruf Amin, the vice presidential running mate of President Widodo, was cited by Tempo.co as saying his side did not cheat and wanted a transparent election.

In 2014, the constitutional court rejected an attempt by Prabowo, as he is usually known as in Indonesia, to overturn his loss in the election over claims it was tainted by cheating.

Political analyst Kevin O’Rourke said voter list issues had been problematic for some time, due partly to dysfunction between different authorities overseeing the data.

However, he added, “Organizing systematic fraud on a scale capable of swaying the outcome would be virtually impossible to conceal, given the extensive role of election monitoring agencies, observer groups and witnesses from each contestant’s campaign.”

Nonetheless, O’Rourke said that if one side did challenge the result in the constitutional court, a ruling would not come until Aug. 8, ushering in several months of uncertainty.