Lula, who led Brazil from 2003 to 2010, has emerged as a leading contender to face the far-right incumbent next year after a Supreme Court justice annulled his convictions on Monday and reinstated his political rights.
In his first comments since the ruling, Lula, 75, gave a scathing take-down of Bolsonaro's management of the economy and signature policies.
He was especially biting on Bolsonaro's handling of the coronavirus pandemic, which has killed more than 266,000 people in Brazil – the second-highest death toll worldwide, after the United States.
"This country has no government," Lula told a news conference. "This country doesn’t take care of the economy, of job creation, wages, health care, the environment, education, young people."
Bolsonaro has repeatedly downplayed the new coronavirus, flouted expert advice on containing it and fuelled vaccine scepticism.
"Don't follow any imbecile decisions by the president of the republic or the health minister: get vaccinated," Lula said.
The former president "managed to sound both serene and angry, both radical and conciliatory," said FRANCE 24's correspondent Tim Vickery. "This is an experienced politician showing that he is still at the top of his game."
Lula, a former metal worker and union leader, led Brazil through an economic boom and is remembered for social programmes that helped lift tens of millions of people from poverty.
Recent opinion polls suggest he is the best-placed politician to unseat Bolsonaro in the October 2022 elections.
>> Lula's return opens door to Bolsonaro showdown in polarised Brazil
But he remains a highly controversial figure after being sentenced to a total of 26 years in jail on corruption charges stemming from a sweeping investigation into a scheme in which top politicians and business executives systematically siphoned billions of dollars from state oil company Petrobras.
He spent more than 18 months in prison, before being released in 2019 pending appeal.
Campaign launch in all but name
Lula called himself the victim of "the biggest judicial lie in 500 years," repeating his claim that the graft charges against him were fabricated to sideline him from the 2018 presidential race, paving the way for Bolsonaro's victory.
He said he planned to "fight tirelessly" for Brazil and that he wanted to resume touring the country once he is vaccinated against Covid-19 next week.
But he declined to say whether he would run in the elections, saying, "My head doesn't have time to think about a 2022 candidacy now."
Still, "his speech was a campaign launch" in all but name, according to political analyst Creomar da Souza, of the consulting firm Dharma.
"He presented his project for the country, which involves a lot of references to his legacy as president," da Souza told AFP.
Lula is still seen as a hero on the left, which argues he was the victim of a conspiracy.
Supporters point to the fact that the lead judge in the anti-corruption probe that ensnared him, Sergio Moro, went on to accept the post of justice minister under Bolsonaro, and that hacked phone messages suggest Moro conspired with prosecutors to ensure Lula was sidelined.
Lula still faces a series of corruption and influence-peddling charges, including the ones he was jailed for, which will now be transferred to a federal court in Brasilia.
But it may already be too late for other courts to rule him out of the 2022 race, said FRANCE 24's Vickery: "In order for him to lose his political rights again he would have to be convicted and then lose again on appeal, and there may not be time for that to happen before the next presidential campaign in October of next year."
(FRANCE 24 with AFP)