Brazilian President Dilma Rousseff has been sworn-in for a second term, BBC reported.
Ms Rousseff, 67, has vowed to extend social welfare programmes that have lifted millions out of poverty.
"Never have so many people entered the middle class, never have so many Brazilians had so much access to education," she said.
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.
But her government is facing a major corruption scandal involving state-run oil company Petrobras.
Thousands of Ms Rousseff's supporters turned up for the swearing-in 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.
'Long-term process'
"In my first term, 36 million people were lifted 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 Brazilian people have understood this is a long-term process. I have been re-elected to carry out the big changes Brazil needs," she added.
The motto for her second term will be "Brazil, a country of education," Ms Rousseff announced.
Ms Rousseff 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.
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.
Representatives from more than 130 countries attended the swearing-in ceremony. Vice-President Michel Temer was also sworn into office.
The presidents of Venezuela, Nicolas Maduro; Uruguay, Jose Mujica; Bolivia, Evo Morales, and Chile, Michelle Bachelet were among the leaders present.
The United States and China were represented by their Vice-Presidents, Joe Biden and Li Yuanchao, according to Agencia Brasil.
'Kickback scheme'
Ms Rousseff won 51.6% of the vote in October, edging out centre-right candidate Aecio Neves, who took 48.4%.
She said "dialogue" would be her top priority after a bitterly fought campaign.
But she 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.
She admitted that Petrobras must be better managed, but said the company was bigger than any crisis.
"We have many reasons to defend Petrobras 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," Ms Rousseff added.
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.