Senior judges in Brazil voiced concern on Saturday over the detention of former president Luiz Inacio Lula da Silva, even as they threw their support behind the sweeping corruption investigation that threatens to topple his embattled successor.

Lula's three hours of questioning in police custody on Friday was the highest profile development in the two-year-old probe focused on state oil firm Petrobras.

The detention of the 70-year-old Lula, Brazil's first working-class president, shocked his supporters. His 2003-10 government helped lift an estimated 40 million Brazilians out of poverty.

Supporters and opponents of Lula clashed outside his home on the outskirts of Sao Paulo after he was detained by police early on Friday.

Supreme Court Justice Marco Aurelio Mello said on Saturday that "nothing justified the use of force" when police picked up Lula unannounced from his apartment.

Even Justice Gilmar Mendes, who has publicly said there is strong evidence the ruling Workers' Party used graft proceeds to fund electoral campaigns, called Lula's interrogation in police custody a "delicate" situation.

The federal judge who ordered Lula to be brought in for questioning, Sergio Moro, said in a statement on Saturday that steps were taken to protect Lula's image during the operation, and he expressed regret that it sparked violence.

Speaking to his supporters at Workers' Party headquarters in Sao Paulo after being released on Friday, Lula said investigators were "disrespectful of democracy" and that he "deserved respect".

Lula has repeatedly denied any wrongdoing either while in office or since.

President Dilma Rousseff flew to meet with Lula on Saturday in a show of support for her predecessor and political mentor. She called his detention "unnecessary".

But the public prosecutors said in a statement that "Lula is owned no more respect than any other Brazilian citizen, as he holds no power that puts him beyond the reach of the investigation".