Former President Luiz Inacio Lula da Silva launched his presidential campaign on Saturday, vowing to rebuild Brazil following the "irresponsible and criminal" administration of far-right incumbent Jair Bolsonaro.

The campaign launch completed Lula's (2003-2010) remarkable political comeback, four years after the 76-year-old leftist icon was imprisoned on controversial corruption charges, that the Supreme Court of Brazil had annulled.

"We're ready to work not only to win the election on October 2, but to rebuild and transform Brazil, which will be even more difficult," Lula da Silva told a rally in Sao Paulo.

He said Bolsonaro, whom he did not name, had turned Brazil into a "pariah" through polarizing policies, attacks on democratic institutions, and rampant destruction of the Amazon rainforest.

"We need to change Brazil once again... We need to return to a place where no one ever dares to defy democracy again. We need to send fascism back to the sewer of history, where it should have been all along," he added, urging "all democrats" to join him.

Lula da Silva is eyeing the seat to replace the far-right Jair Bolsonaro. Da Silva's election would stage a remarkable return to political life after being jailed for corruption for 4 years.

Da Silva left the office as the most popular president in Brazilian history - his return to elections does not come as a surprise, though his lead in the polls has shrunk.

Since March last year, Da Silva was unofficially declared as running for election, particularly when the Supreme Court annulled corruption convictions that had banished him from political life. 

Two days later after the annulment, he slammed Bolsonaro's policies, and asserted that he still feels "young enough to fight."

Lula, between the years 2003 and 2010, left with an approval rating of 87%, especially since he lead an economic boom that lifted around 30 million Brazilians out of poverty.

At one point, Lula looked set to beat Bolsonaro in the first-round vote on October 2, without needing a runoff on October 30.