Brazil's unpredictable election took another twist on Sunday, with left-leaning President Dilma Rousseff being forced into a runoff race as expected, but against a conservative challenger who only surged in the final week of the campaign.

Rousseff will face Aecio Neves in the October 26 runoff vote, required as no single candidate won an outright majority. With over 99 percent of the vote counted, the president had won 41.5 percent against Neves's 33.6 percent.

"The fight continues and we shall win," Rousseff told supporters in Brasilia following the results.

The results indicated a stunning fall for former environment minister Marina Silva, who in late August held a commanding lead in polling, but took just 21 percent on Sunday.

FRANCE 24’s Bruce Douglas in Rio de Janeiro described the results as “a real surprise”.

Rousseff’s aggressive campaigning eviscerated the support of Silva, who only entered the race in mid-August following a plane crash that killed her Socialist Party’s original candidate.

It was thought Silva would tap into the widespread disdain Brazilians hold for the political class – anger that boiled over into roiling, nationwide anti-government protests last year. Opinion polls taken just after the demonstrations over a year ago indicated Silva was among the few political figures unscathed, given her squeaky clean reputation amid what voters say is a sea of corruption.

But Silva has not withstood a barrage of attacks labeling her as indecisive and without the mettle needed to lead the globe’s fifth-largest nation – the message pounded on by Rousseff and the other top opposition candidate, Aecio Neves of the Social Democracy Party.

“Marina Silva tried but was not able to convey her message of change. She’s only responding to attacks,” said Paulo Sotero, director of the Brazil Institute at the Woodrow Wilson International Center for Scholars in Washington. “We’ve seen negative campaigning before, but never at this level of ferocity.”

'Fear campaign' against Silva

Rousseff’s support rose to 46 percent in a survey released hours before the vote. But even the leader said it was unlikely she could push through to win the absolute majority required to avoid a second-round election on October 26.

“I’m not operating with that idea; I’m working with the idea there will be a runoff,” Rousseff said just before casting her vote in southern Brazil, where she lived for many years and first entered politics.