Mr Subianto held a live televised press conference on Tuesday to
declare his withdrawal
But Mr Subianto told reporters there was massive systemic fraud and vowed to challenge the results in court.
He added that his camp would not resort to violence.
Most "quick counts" after the 9 July election had placed Mr Widodo, the popular Jakarta governor, in the lead.
But his rival Mr Subianto, a former military general under Suharto, said earlier that other polls showed he had won.
The Election Commission has since been tallying votes. Outgoing President Susilo Bambang Yudhoyono has urged both parties to respect the result.
Security is tight for the announcement, with more than 250,000 police officers on duty across the nation.
There are fears the result could trigger violence between rival groups of supporters.
Both candidates have raised concerns about voting irregularities during the past two weeks, as ballots from nearly 500,000 polling stations have been tallied.
Reliable polls point to a win for Mr Widodo, also known as Jokowi, by about five percentage points.
Official results from the sub-district and provincial levels that have been released so far also indicate Mr Widodo has won by between three and five percentage points.
But Mr Subianto's camp have said they will not accept official results until allegations of cheating are probed.
Mr Subianto told reporters on Sunday that if the Election Commission did not investigate possible voting fraud, that was a "crime" that "very much calls into question the legitimacy of the whole process".
He called on the Election Commission to delay announcing the result - a request the commission rejected.
BBC
