Environmental campaigners and Green party activists heralded a breakthrough today after a former rubber-tapper from the Brazilian Amazon who rose to be a world-famous rainforest defender became the central figure in the second round of the country's presidential election.

Marina Silva, who was raised in the Amazon state of Acre and was illiterate until the age of 16, failed to make the second round but came away with 19% of the vote, far higher than pollsters had expected.

President Luiz Inácio Lula da Silva's chosen candidate, Dilma Rousseff, meanwhile missed out on a first-round majority, polling a disappointing 46.7%.

With Rousseff's main rival, the Social Democrat José Serra, taking just under 33% of votes, Silva became the king or queen maker in the 31 October runoff when Serra and Rousseff do battle.

"It is a spectacular result," Alfredo Sirkis, the Green party's president in Rio de Janeiro, told the Guardian.

Sirkis said the record vote meant the Green party would be able to force debate on crucial environmental issues in the lead up to the second round. Such issues included controversial changes to Brazil's forestry code, which environmentalists claim will further damage the Amazon rainforest, and Brazil's commitments on climate change in Copenhagen.

The O Dia newspaper in Rio de Janeiro, where Silva came second with 31.52% of the vote, described a "green tsunami" in its front-page headline.

"Marina Silva's face will not be on the ballot on October 31 but her electoral ghost will decide the second round," the newspaper said. "She has become the central figure in this campaign," said Altino Machado, an Amazon journalist and blogger who has known Silva since the late 1970s.

Machado said Silva's success showed "the people's desire for change".

"There are nearly 20 million Brazilians who understood her positions and who voted for her as an alternative … believing in a utopia, a possible country."

Machado said the result would allow the country to debate not just Brazil's "commitment to development and the wellbeing of the population" but also its "commitment to environmental preservation".

Within minutes of last night's result becoming clear, Serra was making public overtures to Silva.

He had originally tried to recruit her as his presidential running-mate.

"I want to congratulate Marina for her significant vote," Serra said in Sao Paulo, adding that the Green party candidate had "contributed to democracy in Brazil". He said: "Today I am all thanks."

Members of president Lula's Workers' party will also turn their attentions to Silva, who walked away from her job as his environment minister in 2008 after reportedly clashing with Rousseff.

Brazilian newspapers today reported that Lula would now take direct control over Rousseff's campaign, in order to ensure her election.

Sirkis said it would take "up to 15 days" for the Green party to decide which of the candidates, if any, it might support.

"We will meet the two candidates and then hold our convention [to decide]," he said.

"There is clearly an anxiety in the Brazilian press to know if we will support Serra or Dilma. [But] they will have to wait a while. This decision cannot be taken this week."

Machado, who used to act alongside Silva in an Amazon theatre group and was with her on Sunday, said it was unclear which way his friend would lean.

"This is now the great mystery of Brazilian politics over the next few days: what will Marina do?" he said.