A bomb has rocked the offices of Indonesia's election commission, delaying the announcement of the winner of this month's presidential poll.

Dozens of police including bomb squad officers evacuated the premises in Central Jakarta after the blast in a women's toilet. Police said no one was hurt and damage was slight but the incident heightened security fears as the country prepares for a second round of voting in eight weeks' time.

According to partial returns, ex-general Susilo Bambang Yudhoyono is certain to win Indonesia's first direct presidential poll but without the 50 percent needed to avoid a September 20 runoff between the top two candidates.

With about 85 percent of the vote counted, Yudhoyono had 34 percent compared to 26 percent for incumbent Megawati Sukarnoputri and 22 percent for another ex-general, Wiranto.

Wiranto, who stood for the Golkar party of former dictator Suharto, has challenged his likely elimination from the race. He has said the vote was flawed and his supporters say they may go to court to contest the result.

Any legal challenge would further heighten political uncertainty in Indonesia, which has been preoccupied with elections for almost the entire year. Separate polls for national and local legislatures were held on April 5, with Golkar topping the national vote.

Vice-president Hamzah Haz criticised security services after the blast. "The security and intelligence services should have detected [this] earlier to prevent this incident from happening," he told reporters. "It is not the KPU [election commission] which was careless. We knew these days would be critical so we should have intensified security."

Election officials were completing a manual count of district vote tallies from the 32 provinces. One of them, Hamid Awaluddin, said the counting would resume at 3pm but there was no information on when it would end. "I am saddened that this incident had to take place because we have gone through a lengthy and difficult process and we are almost finished," said senior election commission official Chus'nul Mariyah.

Wiranto, a former military chief who has been accused of condoning major rights violations during his tenure, complained of irregularities in the handling of ballot papers which were initially deemed spoilt.

Millions of ballot papers were not counted when voters inadvertently punched two holes in the folded papers. Just before the polls closed on July 5, the election commission ordered that such votes should be considered valid.

Former US president Jimmy Carter, one of hundreds of foreign poll monitors, had said various problems with the ballot did not appear to favour one candidate over another.

Yudhoyono called for a clean fight in the second round. He has complained of smear tactics spread by SMS messages in the world's largest Muslim-populated nation, which falsely allege that he is secretly a Christian.

"If we engage in a healthy competition, the people will get a good winner," he told hundreds of supporters. With Wiranto all but certain to be eliminated, both the Yudhoyono and Megawati camps have been putting out feelers about securing Golkar's support.

Golkar party chief Akbar Tanjung said Saturday his party has received a "concrete" coalition offer from Megawati. The party has not yet officially decided which camp to support.

Surveys show that party endorsements could in any case be of limited help in swaying voters on September 20. Almost a third of Golkar supporters voted for Yudhoyono, a former security minister, rather than for their official candidate Wiranto.