Brazil's newly elected leader, Dilma Rousseff, says she will work to lift millions of people out of poverty when she replaces the popular outgoing president.

Rousseff, 62, will succeed President Luiz Inacio Lula da Silva, who was barred by Brazil's constitution from seeking a third consective term.

She beat out centrist rival Jose Serra in a run-off vote Sunday. With 99 per cent of the ballots counted, Rousseff had 55.6 per cent compared to 44.4 per cent for Serra, Brazil's electoral court said.

She will become the country's first female president when she is sworn in on Jan. 1.

Speaking shortly after her victory, Rousseff said she promised to work to eradicate poverty and create new opportunities.

"We cannot rest while Brazilians are going hungry," she said.

Tears came to her eyes as she thanked the outgoing president for his support.

"It's a challenging and difficult task to succeed him, but I will know how to honour his legacy," she said of Silva, known as Lula. "I will know how to advance and consolidate his work."

Rousseff was a career civil servant who has never held an elected office. She's a former Marxist guerrilla who was jailed and tortured for three years in the early 1970s for fighting against Brazil's dictatorship.

She served as energy minister from 2003 to 2005, when she became the president's chief of staff.

"I know that a leader like Lula will never be away from his people," she said. "I will always be able to knock on his door and I'm sure that it will always will be open."

Silva took office with a background as a leftist labour leader, but governed from a moderate perspective. Under his leadership, the economy grew and Brazil weathered the global financial crisis better than most nations.