The leftist was freed after the Supreme Court ruled on Thursday that convicted criminals can only be imprisoned after they exhaust their appeals.
Lula is appealing his conviction of corruption and money laundering in connection with the purchase of a beachfront apartment in Sao Paulo state – accusations he denies and says are politically motivated.
“The only fear that I have is to lie to working people. All I have is thanks to my mother, who was born and died being illiterate,” he told supporters.
“I doubt that Jair Bolsonaro and Sergio Moro can sleep with a calm conscience as I do. And I want to tell them: I am back!,” the former president said, referring to the current Brazilian president and justice minister, who had sentenced him.
Moro, who was a judge in 2018 and handed Lula eight years and 10 months in jail, was later made justice minister by Bolsonaro, a conservative.
Lula had been found guilty of accepting bribes from engineering firms in return for government contracts.
Writing on Twitter on Saturday, he said: “If we work well, in 2022 (when elections are expected to take place) the so-called left wing that Bolsonaro is so afraid of will defeat the ultra-right in that country.”
In his first speech on Friday, Lula pledged to fight to establish his innocence and excoriated what he called the “rotten side of the judicial system” for “working to criminalise the left”.
Brazil’s first working-class president, he led Brazil from 2003 and 2010 and left office with high popularity ratings thanks to social policies that raised millions from poverty.
He was favoured to win the 2018 presidential election, but his imprisonment barred him from running.
Earlier on Saturday, Bolsonaro called Lula a “scoundrel”.
Writing on Twitter, the president said: “Lovers of freedom and good, we are the majority. Without a north star and a directive, even the best troops become a band shooting at all sides, including friends.
“Do not give ammunition to the scoundrel, who is momentarily free but full of guilt.”