Dilma Rousseff, 63, was inaugurated Saturday in Brasilia as the first female president in the history of Brazil, dpa reported.
She succeeded her mentor Luiz Inacio Lula da Silva, who left office with record popularity but could not stay on due to constitutional limits on consecutive presidential terms.
Rousseff was inaugurated in lower house of the Brazilian Congress.
"I promise to keep, to defend and to abide by the constitution, to observe the law, to promote general welfare and to sustain Brazil's union, integrity and independence," Rousseff said.
Her oath met with applause from 800 invited guests, including more than 20 foreign heads of government.
Dressed in an elegant ivory-coloured dress, she had arrived at the ceremony in the company of her only daughter, Paula, in a closed vehicle. The heavy rain prevented her from using an open car for the parade.
Rousseff, who trained as a guerrilla in the 1960s and was arrested and tortured under the dictatorship that ruled Brazil 1964-85, earned the right to govern the world's eighth-largest economy by defeating social-democrat Jose Serra in a runoff on October 31.