Dilma Rousseff has been elected president of Brazil, succeeding Luiz Inacio Lula da Silva, electoral officials have confirmed.
Ms Rousseff, 62, who had never before held elected office, becomes the country's first woman president.
She has enjoyed the full support of President Lula, who is leaving after two terms with record popularity.
Ms Rousseff has promised to continue policies that have fostered years of strong economic growth.
The Superior Electoral Court declared her the winner with 92% of the votes counted.
She has won 55% of those votes, with her main rival Jose Serra taking 46%.
This second round of voting was forced after Ms Rousseff fell short of the 50% needed in the 3 October first round. She won 47% to Mr Serra's 33%. More than 130 million voters were due to take part in the polls.
'New phase'
The career civil servant is expected to broadly continue Mr Lula's left-leaning policies, with an emphasis on raising government efficiency, expanding the role of the state in some sectors such as mining, and upgrading the country's decrepit infrastructure.
A bill aimed at reforming Brazil's Byzantine tax system is likely to be Ms Rousseff's first major legislative effort after taking over from President Lula on 1 January.
She can count on strengthened majorities for the ruling coalition in both houses of Congress to help ease the task of pushing her legislative agenda.
Ms Rousseff flashed a victory sign and gave a big smile to photographers after casting her vote in the southern city of Porto Alegre.
"Tomorrow we begin a new phase in our democracy", she said.
"I will govern for everyone, speak for all Brazilians, without exceptions".
After casting his vote in Sao Paulo, Mr Serra of the Social Democratic Party said that after eight years of government by the Workers Party, Brazil needed change.
But he said he had faced an "uneven battle", in apparent reference to the booming economy that has boosted the popularity of President Lula and his preferred successor.
Jose Serra, 68, is a former governor of Sao Paulo, Brazil's most populous state, and a former health minister.
Mr Lula has been active and highly visible in Ms Rousseff's election campaign. He has to step down after completing the maximum allowed two consecutive terms.
BBC News
