The ex-general who lost Indonesia's presidential election to Joko Widodo will challenge the result in court, his campaign team said on Wednesday, a move that could spell weeks of uncertainty for the country.

Widodo, the reform-minded governor of Jakarta whose election is seen as a break from the rule of Suharto, was named the winner on Tuesday after results showed he resoundingly defeated his only rival Prabowo Subianto.

Before the result was announced, Prabowo - who was a senior general in the years of authoritarian rule - angrily announced he was withdrawing from the election process.

Prabowo, who had earlier claimed victory in the July 9 election, accused his opponent of cheating in the vote count.

Members of his campaign team indicated on Tuesday that he would not challenge the result in the Constitutional Court, as had been widely expected, because he was no longer a participant in the election. But in a surprise announcement on Wednesday, a Prabowo spokesman insisted that the candidate had not withdrawn from the whole election process, only the vote count - meaning he could still contest the result and planned to do so.

Analysts do not expect a court challenge to succeed, given the size of Widodo's victory - he won by six percentage points or about 8.4 million votes - but it nevertheless signals weeks of uncertainty, as the court will likely issue a ruling on Aug 21.

Prabowo team spokesman Tantowi Yahya announced the decision to contest the result and said a challenge would be filed within three days.

He said the challenge would be directed at the election commission, which Prabowo has accused of mishandling the vote, adding his side considered 21 million votes to be in dispute.

Prabowo's brother Hashim Djojohadikusumo, a wealthy businessman who provided financial backing for the campaign, added: "We are looking for justice. ... We are expecting some fairness."

He also urged foreign leaders not to congratulate Widodo, as "the legal process has not ended yet". US Secretary of State John Kerry and Australian Prime Minister Tony Abbott are among those who have already sent congratulations.

Widodo unperturbed

Widodo seemed unperturbed by his rival's challenge and returned to his duties as the Jakarta governor at city hall on Wednesday. He will not be inaugurated as president until October.

He said preparations for his new job were in progress and a special office to help with the transition had been set up.

Newspapers welcomed his victory, with the major Indonesian-language paper, Kompas, showing a photo of a grinning Widodo alongside his running mate Jusuf Kalla, under the headline: "It's time to move together".

A Constitutional Court official said that if Prabowo's team filed the appeal by Friday, hearings would begin on Aug 6 and a ruling would be delivered on Aug 21.