Despite an initially glum mood at her campaign headquarters as results came in, Rousseff voiced optimism and said she could deliver the change Brazilians want after street protests swept the country last year - while preserving the achievements of recent years, such as low unemployment.

"My second term will be much better than my first," she said.

Neves' strong showing seems likely to spark a rally in Brazil's stocks and currency on Monday. Investors have in recent months bid up assets every time a change in government seems more likely.

"The next few weeks are going to be weeks of intense volatility in the markets," said Paulo Rabello de Castro, chairman of SR Rating, a Brazilian rating agency. "But I really wish I had bought Brazilian assets last week."

After growing at more than 4 percent a year during a commodities-fueled boom last decade, Brazil's economy has averaged less than 2 percent growth under Rousseff and many on Wall Street and in Sao Paulo have made no secret of their desire for more market-friendly policies.

Andre Cesar, a political analyst, said Neves appeared to have an even chance at victory. "(Neves) has turned into a very difficult adversary for Rousseff. He has gained muscle and a new energy," Cesar said.

Rousseff counts on a bedrock of support among the working class, thanks to generous social welfare programs and the record of her ruling Workers' Party in reducing one of the world's biggest gaps between rich and poor.

Support from her predecessor Luiz Inacio Lula da Silva, who remains by most measures Brazil's most popular politician, will also boost her in the runoff race.

"What's at stake is continuity," said Ana Augusta de Medeiros, a 71-year-old voter in Rio de Janeiro. "I hope they will continue working on behalf of the poor."

Yet others yearn for a change, pointing to poor health care and education services and infrastructure bottlenecks that have made Brazil one of the world's most expensive and difficult places to do business.

"Dilma already tried. The things that she promised she did not complete," said Rosilene Silva de Jesus, 29, who voted for Silva in Sao Paulo.

Because of the topsy-turvy race, campaigning was noisier than usual in a country where the electoral process at times feels more like a carnival.

Candidates employed armies of pamphleteers and flag-wavers at street corners, while campaign jingles, often composed by celebrity musicians, blasted from sound cars and televisions.

This year's frenzy was disrupted when Campos' plane crashed in bad weather along the coast just south of Sao Paulo. Silva rode a wave of public sympathy and her own inspiring, up-from-nothing life story to first place in the polls in early September, but then stumbled badly at the end.