"I fear nothing," he said, after being interrogated on Friday over allegations that he received millions of dollars in illegal payments.
Lula, a left-wing icon, left office in 2011.
His Workers' Party has been hit hard by the long-running scandal.
His brief detention led to clashes outside his home near Sao Paulo.
After his interrogation Lula told reporters he was the victim of a "prejudice as a working-class man", adding: "I escaped death from hunger when I was five... I was the best president to govern this country, the poorest of this country re-elected me because they could eat now."
He also said there had been no need for police to detain him.
"If they wanted to hear from me, they only had to call and I would have gone, because I owe nothing to anyone and fear nothing."
This was echoed by his successor, current President Dilma Rousseff, who said it had been "unnecessary" to take him into custody.
The Petrobras inquiry, known as Operation Car Wash, has led to criminal proceedings against dozens of business executives and Workers' Party figures accused of taking part in the multibillion-dollar corruption scheme.
The Workers' Party has held the Brazilian presidency since 2003, both under Lula and Ms Rousseff.
In the latest operations, police enforced 33 search and 11 detention warrants in the states of Rio de Janeiro, Sao Paulo and Bahia, officials said.
Lula, 70, is suspected of receiving about 30m reais ($8m; £5.6m) in speaking fees and donations to his charity, the Lula Institute.
His home in Sao Bernardo do Campo was among the premises targeted, as was the headquarters of the institute in Sao Paulo.
Prosecutor Carlos Fernando dos Santos Lima said: "Among the five largest donors to the Lula Institute, 60% of all the donations were paid by the five largest businessmen involved in the Car Wash scandal."
A police statement quoted by Reuters news agency said: "There is evidence that the crimes enriched him and financed electoral campaigns and the treasury of his political group."
However, no charges have been brought against Lula so far.
The institute said in a statement (in Portuguese) the "violence" against the former president was "arbitrary, illegal and unjustifiable", as he had been co-operating with the investigations.
The Workers' Party called the operation a "political spectacle".