"I promise to maintain, defend and fulfill the Constitution, observe the laws, promote the good of the Brazilian people, hold up the unity, integrity and independence of Brazil," Lula da Silva said.

In his previous years as Workers Party (PT) president from 2003-2010, the former union leader lifted millions of Brazilians from poverty during a commodity boom that buoyed the economy.

Now, he faces the daunting challenge of improving Brazil's stagnant economy while also uniting a country that has become painfully polarized under Bolsonaro.