Brazilian President Dilma Rousseff has been sworn in for a second term.
In her inaugural address, Ms Rousseff, 67, vowed to extend social welfare programmes that have lifted millions out of poverty.
She also promised to investigate a major corruption scandal involving state-run oil company Petrobras.
Ms Rousseff, a left-wing economist who was arrested and tortured under military rule, was re-elected by a narrow margin in a run-off vote.
Thousands of Ms Rousseff's supporters turned up for the ceremony in the capital, Brasilia.
Many were wearing the red colour of Ms Rousseff's Workers' Party (PT), which has been in power since 2003.
Analysis: Julia Carneiro, BBC Brasil
While most of the country woke up drowsy after the New Year celebrations, Brazil's capital rose early to prepare for the solemn inaugural events that launched President Dilma Rousseff into the start of her second term.
Ms Rousseff's new mandate starts amid a grim mood with problems ranging from a stagnating economy to the major corruption scandal hitting Brazil's oil giant Petrobras, once considered the jewel in the Brazilian crown.
But even with this atmosphere, and after a narrow victory that divided the country, Ms Rousseff went ahead and announced a controversial cabinet.
Critics say she has distributed posts to please the parties of the government's coalition rather than appointments based on political expertise.
It seems to be the old way of doing politics in Brazil.
Ms Rousseff travelled in an open-top car from the official residence, the Alvorada Palace, to the National Congress building, waving to thousands of people who lined the streets.
"We have lifted 36 million people from extreme poverty," she told lawmakers in the Congress building.
"It is time to pursue new goals. Brazilians want high-quality health and education services, security and that corruption be tackled.
The motto for her second term will be "Brazil, a country of education," Ms Rousseff announced.
She also promised to get the economy growing again, but said millions of jobs had been created in Brazil over the past four years "despite difficult circumstances" in the world economy.
Representatives from more than 130 countries attended the swearing-in ceremony, which also saw Vice-President Michel Temer take his oath.
The presidents of Venezuela, Nicolas Maduro; Uruguay, Jose Mujica; Bolivia, Evo Morales, and Chile, Michelle Bachelet were among the leaders present.
US Vice President Joe Biden and his Chinese counterpart Li Yuanchao also attended, according to Agencia Brasil.
Ms Rousseff won 51.6% of the vote in October, edging out centre-right candidate Aecio Neves, who took 48.4%.
Her government faces a challenge from the ongoing Petrobras scandal, which emerged during the election campaign.
Ms Rousseff served as chair of the Petrobras board for seven years until 2010, but has denied any knowledge of wrongdoing.
So far, 39 people have been indicted on charges that include corruption, money laundering and racketeering.
They are accused of forming a cartel to drive up the prices of major Petrobras infrastructure projects and of channelling more than $3.9bn (£2.5bn) money into a kickback scheme at Petrobras to pay politicians.
"Corruption must be wiped out," Ms Rousseff said in her swearing-in speech, adding that Petrobras must be better managed but also defended "from its internal predators and also from its external enemies".
"That is why we will carry out a thorough investigation and avoid that a similar scandal happens again," she said.
In her first four-year term, Ms Rousseff enjoyed the benefits of the social policies initiated by her predecessor, Luiz Inacio Lula da Silva, who was constitutionally barred from seeking a third term.
But economic growth declined over the past two years and discontent with spending for the 2014 football World Cup and the 2016 Olympics in Rio led to months of street protests.