Dressed in a black blazer and T-shirt, da Silva spoke from a stage outside the union near Sao Paulo that he once led and that served as the base for his political career. The crowd of red-clad supporters cheered and waved flags.
"We are going to do a lot of fighting. Fighting is not one day on, then three months off, then back. Fighting is every day," said da Silva, a 74-year-old who promised to bring the energy of a 30-year-old to the streets.
In his 45-minute speech, he spoke briefly of conservative President Jair Bolsonaro, who won the 2018 election after da Silva's corruption conviction barred him from running. Da Silva said Brazilians must accept the results of the democratic election and work to defeat the "ultra-right" in 2022.
He also called for solidarity with fellow South American countries and lambasted U.S. President Donald Trump, saying his border wall plan is unacceptable and aimed at keeping out poor people.
"Trump should resolve Americans' problems and not bother Latin Americans. He wasn't elected to be the world's sheriff," said da Silva, who in a Twitter post Friday backed U.S. Sen. Bernie Sanders' presidential campaign.
Brazil's Supreme Court on Thursday ruled that a person can be jailed only after all appeals to higher courts have been exhausted. Da Silva was released the next day, after 19 months imprisonment.
He is still appealing his conviction related to the alleged purchase of a beachfront apartment and remains entangled in other cases. He was also convicted by a lower court judge in a case involving ownership of a farmhouse in Atibaia, outside Sao Paulo.
If he loses his appeals in either conviction, he could be locked up again.
Da Silva has denied any wrongdoing and accused prosecutors and Sergio Moro, then a judge and now justice minister, of manipulating the case against him.
Moro said on Twitter earlier that the Supreme Court's decision should be respected, but Congress could alter the constitution to change when convicted criminals start serving their sentences.
Some Brazilian groups organized demonstrations in dozens of cities in support of the Bolsonaro administration, but turnout was low.
The president's son, Eduardo Bolsonaro, said on Twitter that da Silva's release will prompt people to set aside differences and unite against the Workers' Party, a sentiment that helped carry his father to the presidency.
Jair Bolsonaro had refrained from commenting on da Silva, but when asked by journalists about the case Saturday, the president responded: "He is free, but he still has all his crimes on his back."
Da Silva said he had a message for his opponents in power: "I want to say to them: I'm back."
Associated Press writer Mario Lobão reported this story in Sao Bernardo do Campo and AP writer Diane Jeantet reported from Rio de Janeiro.
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