By Reese Ewing
SAO PAULO (Reuters) - 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 Petroleo Brasileiro SA (Petrobras).
The detention of 70-year-old Lula, Brazil's first working class president and a hero to millions of Brazilians, shocked many in the South American nation. His 2003-2010 government helped lift an estimated 40 million Brazilians out of poverty and raised the nation's international profile.
Supporters and opponents of the former union leader clashed outside his home on the outskirts of Sao Paulo after he was detained by police early on Friday. There were also noisy protests outside the police offices where he was questioned.
Supreme Court Justice Marco Aurélio Mello told CBN Radio 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 in O Estado de S.Paulo newspaper.
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 that investigators were "disrespectful of democracy" and abusing their authority.
He has repeatedly denied any wrongdoing either while in office or since.
President Dilma Rousseff flew from Brasilia on Saturday morning to meet with Lula in his apartment in Sao Bernardo do Campo, in a show of support for her predecessor and political mentor. Rousseff said on Friday it was "unnecessary" to take Lula into custody because he had already voluntarily answered investigators' questions.
Before Friday, some analysts had questioned whether the crackdown on graft could be at risk of fizzling out after the justice minister stepped down amid a barrage of criticism from the ruling Workers Party over the investigation.
But Supreme Court justices were unambiguous in support for prosecutors pursuing their investigations into Brazil's biggest ever corruption scandal.
Justice Rosa Weber late on Friday rejected a request by Lula's lawyers to suspend investigations into real estate assets and the ex-president's institute, which police allege he and his family likely used to launder money stolen from Petrobras.
Even Justice Mello, who was critical of Lula's arrest, said the evidence uncovered by prosecutors was "very serious" and deserved further investigation.
The pressure on Rousseff and Lula shows no sign of abating. Local media has reported on a spate of new plea bargains by senior politicians that could further ensnare the government.
IstoE newsmagazine on Thursday published alleged testimony by Workers' Party Senator Delcidio Amaral claiming that Rousseff and Lula both directly benefited from corruption. He alleged Rousseff had tried to hamper the investigation.
The president strongly denied the reported accusations on Friday.
Epoca weekly magazine, meanwhile, published on Saturday testimony from ex-Congressman Pedro Correa, who also outlined graft at Petrobras stretching back to Lula's government.
Ratcheting up political tensions, opposition lawmakers said on Friday they will block Congressional voting until the Supreme Court clears the way for the impeachment process against Rousseff.
Congress is due to hear the proceedings against the president, on grounds that she broke budgetary guidelines to boost her 2014 re-election campaign, but is awaiting guidelines on how to proceed from the court first.
(Additional reporting by Pedro Fonseca; Editing by Daniel Flynn and Tom Brown)