Near-complete results in Brazil put Dilma Rousseff of the governing Workers' Party well ahead in the country's presidential election.
But with 97.5% of ballots counted, Ms Rousseff is still 3.6% short of the 50% total which would give her an outright win.
She will now have contest a second round at the end of October against the second-placed candidate, Jose Serra.
Ms Rousseff is the favoured successor to President Luis Ignacio Lula da Silva, who has completed two terms.
Analysts say Ms Rousseff ran a careful campaign, benefiting from Mr Lula's widespread popularity and the country's booming economy.
She was a frontrunner for much of the campaign.
Centre-left candidate Jose Serra, of the Social Democratic Party, had pinned his hopes on getting enough votes to force a second round.
President Lula, who is constitutionally barred from standing for a third consecutive term, acknowledged that the poll could go to a run-off.
"The election has two rounds. I have never won an election in the first round. It will be 30 more days of fighting... and let's go to this fight," he said.
Mr Lula stressed, however, that Ms Rousseff, his former chief of staff, was in a strong position to win.
The latest polls published on Saturday suggested Ms Rousseff's attempt to win enough votes to avoid a run-off vote on 31 October would be extremely tight.
O Globo newspaper's prediction had Ms Rousseff winning 51% of the vote, with Mr Serra on 31%; the Folha de Sao Paulo newspaper poll put Ms Rousseff on 50% and Mr Serra on 31%.
Polls have consistently suggested Ms Rousseff would win a second round by a wide margin, but analysts say her position would be strengthened if she could win outright on Sunday.
Brazil, one of the world's most populous democracies, is also choosing local and national representatives.
Maria Silveira, a Rousseff voter in Mr Lula's constituency, Sao Bernardo do Campo, outside Sao Paulo, told the Associated Press news agency: "It only makes sense to vote for the candidate who I know will continue what he started."
But AP quoted 22-year-old student Iracy Silva as saying: "I voted for Serra because he has much more experience than the other candidates."
Voting is compulsory in Brazil.
Polls closed at 2200 GMT (1700 local time), with results coming quickly, thanks to Brazil's electronic voting system.
Ms Rousseff, of the ruling Workers' Party, saw her lead in the opinion polls slip in the final days of campaigning after corruption allegations surfaced involving a former aide.
But her campaign has been boosted by energetic support from Mr Lula.
"I'm convinced the majority of people want continuity from the government," Mr Lula told a rally on Friday.
"That's why I think Dilma will win."
Ms Rousseff, 62, served as Mr Lula's chief of staff from 2005 until this year, and is a career civil servant. Her tilt at the presidency is her first attempt at elected office.
During the 1960s and 1970s she was involved in the armed struggle against Brazil's military rulers, and was jailed for three years.
The 68-year-old Mr Serra is hugely experienced, having served as Sao Paulo mayor, Sao Paulo state governor and health minister under President Fernando Henrique Cardoso Mr Lula's predecessor. He lost the presidential election in a run-off to Mr Lula in 2002.
Two other candidates for the presidency are trailing far behind in the polls.
Marina Silva of the Green Party and Plinio de Arruda Sampaio of the Socialism and Freedom Party are not expected to trouble the frontrunners.