Dilma Rousseff has been sworn in as Brazil's first woman president, succeeding Luiz Inacio Lula da Silva, the most popular leader in the country's history.
Ms Rousseff, 62, who has never held elected office, has pledged to follow the path set by President Lula.
He was constitutionally barred from seeking a third consecutive term.
Brazil's economy has grown strongly but it remains one of the most unequal societies in the world.
Ms Rousseff was appointed energy minister in President Lula's government in 2003 and served as his chief of staff from 2005 to 2010.
She was elected in October, defeating the opposition candidate Jose Serra by 56% to 44% in a run-off vote.
She is known to favour a strong state role in strategic areas, including banking, the oil industry and energy.
She has also promised to tackle Brazil's complicated tax system.
Strong growth
Ms Rousseff's inauguration ceremony at the Brazilian Congress began with a ride through the capital, Brasilia, in a Rolls-Royce. Rain prevented the vehicle's roof from being opened.
In her oath, she promised to maintain and defend the constitution, observe the law, promote the wellbeing of the Brazilian people, and sustain the union, integrity and independence of Brazil.
She will later join Mr Lula at the presidential palace, where he will drape her in the green-and-gold Brazilian sash of office.
Ms Rousseff, a former Marxist rebel who was imprisoned for three years in the early 1970s for resisting military rule, has promised to protect freedom of expression and worship, and to honour the constitution.
The BBC's Paulo Cabral, in Sao Paulo, says Ms Rousseff faces significant challenges, public health, education and improving the country's infrastructure.
Brazil's economy is estimated to have grown by 8% in 2010. However, the currency, the real, has risen so high that it is now making Brazil's exports less competitive.
During his two terms as president from 2002, 30 million people were lifted out of poverty - a major reason for his status as Brazil's most popular president, our correspondent says.
During President Rousseff's term, Brazil will host the Rio Plus 20 global environmental summit in 2012; the Fifa World Cup in 2014; and the Summer Olympics in 2016.
