Indonesia's Joko Widodo was on track to be re-elected leader of the world's third-biggest democracy as unofficial results put him in a comfortable lead over firebrand ex-general Prabowo Subianto after voting closed Wednesday across the 17,000-island archipelago.

While official results are not due until next month, a series of so-called "quick counts" by pollsters showed Widodo as much as 11 percentage points ahead.

The vote ended at 1:00 pm (0600 GMT) in Sumatra, although some of the 800,000 polling stations across the volcano-dotted nation remained open late due to delays and long queues.

The quick counts have proven reliable indicators in past elections, but Widodo held off declaring victory.

"We've all seen exit poll and quick count numbers, but we still need to wait for the official results," he told cheering supporters in Jakarta.

His 67-year-old rival -- who lost to Widodo in their 2014 presidential contest and warned he would challenge this year's results if he lost -- insisted that exit polls suggested he was in the lead. He did not cite specific evidence.

"I'm calling on my supporters to keep calm and don't get provoked," he said.

The campaign was punctuated by bitter mudslinging and a slew of fake news online -- much of it directed at the presidential contenders.

"I hope after this that there will be a call for reconciliation because ... we've been living in a very polarised atmosphere," political analyst Gun Gun Heryanto told Kompas TV.