Luiz Inácio Lula da Silva took a swipe at Jair Bolsonaro's rhetoric and environmental policies and referred to the president's adviser, Olava de Carvalho.
"Both Haddad and Bolsonaro have high rejection rates, and in the runoff most Brazilians will be voting against the candidate they dislike the most."
"Bolsonaro is a Brazilian version of Trump," Allana Ferreira, a Brasilia-based journalist told Newsweek. She explained that many people think Brazil needs someone with "a strong hand" to remove the current political class.
The far-right politician was shown to be leading in a recent poll, with 22 percent of the vote.
Lula's conviction leaves the 2018 presidential race wide open, raising chances of a victory by a political outsider.
Lula, an extremely popular two-term president who left office in January 2011, faces another trial on graft charges.
Brazilian President Dilma Rousseff said on Tuesday her vice president was orchestrating a conspiracy to topple her.
The court ruled on Thursday to take a corruption investigation away from a crusading federal judge.
Police said 2,500 people demonstrated outside the presidential palace in the capital of Brasilia.
Critics slammed Lula's return as a desperate attempt by President Dilma Rousseff to shore up support.
Former President Luiz Inacio Lula da Silva is not under investigation.