But Prabowo Subianto's last-ditch bid to overturn the result did little to dampen the enthusiasm of Widodo's supporters, with thousands staging a noisy rally next to a Jakarta monument commemorating Indonesia's proclamation of independence in 1945.
Widodo, the reform-minded governor of Jakarta seen as a break from the autocratic era of dictator Suharto, was named the winner yesterday after results showed he resoundingly defeated Prabowo, his only challenger.
Prabowo, who had earlier claimed victory in the July 9 election, accused his opponent of cheating in the vote count.
Today a spokesman for the ex-general's team said he plans to contest the result at the Consitutional Court, with the challenge directed at the election commission for allegedly mishandling the count.
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Analysts believe the poll was largely free and fair and do not expect a court challenge to succeed given the size of Widodo's victory - he won by more than six percentage points, or about 8.4 million votes.
The challenge would be filed within three days, said Prabowo spokesman Tantowi Yahya, adding his side considered 21 million votes to be in dispute.
Prabowo's brother Hashim Djojohadikusumo, a wealthy businessman who has 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, saying that "the legal process has not ended yet".
Congratulations flooded in nevertheless, from US Secretary of State John Kerry and Australian Prime Minister Tony Abbott, as well as the leaders of neighbouring Singapore and Malaysia.
He said preparations for his new job were "in progress" and a special office to help with the transition had already been set up.