BRASILIA, March 9 (Reuters) - An investigation into a massive corruption scheme centered on Brazilian state oil company Petrobras enjoys overwhelming public support but concern is growing that prosecutors may be over-reaching as they pursue politicians and executives.
The high-profile detention for questioning last week of former president Luis Inacio Lula da Silva drew criticism from the government and federal judges, reigniting debate over the methods used in "Operation Car Wash".
Critics say prosecutors trying to unravel the vast network of bribery and political kickbacks and Judge Sergio Moro, who presides over the case, are unfairly leaking evidence to the media and have left defendants in prison for months without charges as part of a strategy to win plea bargain deals.
Those allegations have intensified in the last few days.
"The detention of Lula has put a lot of stress on the whole investigation," said Rafael Mafei, a law professor at the University of Sao Paulo. "It was a very public incident broadcast live on television and involved an ex-president."
Mafei said Moro's methods test the limits of the law but are warranted by the scale of the graft scheme, a situation he compared to Colombia's war on drug cartels in the 1990s.
"Many Brazilians believe that our existing legal system is poorly equipped to handle such massive illegality that is larger than the justice system itself," he said.
Crucially, Moro's tactics have won the backing of higher courts in dealing with Brazil's biggest ever corruption scandal, in which a cartel of builders overcharged Petrobras for contracts, paying bribes to company directors and kickbacks to politicians.
Prosecutors have struck more than a dozen plea bargain deals and none of them have been denied by the Supreme Court, which has to approve testimony before it can be accepted as evidence.
While some Supreme Court justices questioned the way police arrived unannounced to pick up Lula, they publicly backed the investigation.
Moro has used questioning in custody and lengthy pre-trial detentions to garner information leading to dozens of arrests and the recovery of billions of dollars in bribes and political kickbacks.
Backed by an elite police unit in commando gear, officers arrived at Lula's Sao Paulo apartment last Friday at dawn and took the 70-year-old into custody for over three hours of questioning on suspicion he took bribes paid by engineering firms with graft money.
Lula's lawyer said the measure was based on scant evidence and illegal because the former union leader should have been served a subpoena asking him to make a statement before a warrant was issued to bring him in.
"They arrived by surprise with a disproportionate show of force, intent on making a public spectacle of his detention," his lawyer, Cristiano Martins, told Reuters. "You cannot resort to illegal methods under the pretext of fighting corruption, just by saying the ends justify the means."
Lula, popular leftist leader who was president from 2003 to 2010, went straight to the headquarters of the ruling Workers' Party after his questioning and rallied his followers to take to the streets to defend him.
His protégé President Dilma Rousseff, fighting to survive the scandal amid a severe recession, said Lula's detention was "unnecessary" because he would have answered questions voluntarily.
The Workers' Party said the move was aimed at destroying Lula's legacy as defender of the poor so he cannot run again for president in 2018.
Federal prosecutors insist that they will push ahead with the probe regardless of the consequences.
"The storm will pass. Our only concern is that our investigations not be used politically by one side or the other," said a senior official in the federal prosecutor's office, known as the PGR.
Prosecutors are confident the Supreme Court will accept a plea bargain testimony of Workers' Party senator Delcidio do Amaral, reports of which sent shockwaves through Brazil's political establishment last week.
According to the IstoE news magazine, which had access to the document, Amaral accused Rousseff of trying to interfere in the Petrobras investigation, an impeachable offense, and said her 2014 re-election campaign was funded by graft money.
Lawyers for Marcelo Odebrecht, former CEO of Latin America's largest engineering group, criticized judicial overreach in arguing for his release after six months of pre-trial detention, but Brazil's second-highest court voted against the appeal.
Odebrecht was sentenced on Tuesday to 19 years in prison for bribery, money laundering and organized crime.
Prosecutors hope the stiff sentence will lead one of Brazil's richest men to seek a plea bargain.
Last month, the Supreme Court ruled convicted criminals should start their prison sentence as soon as it is confirmed by the circuit court and not after a lengthy appeals process.
The decision will reinforce Moro's strategy, Mafei said.
The Supreme Court ruling prompted Workers' Party lawmaker Wadih Damous to proposed a bill banning plea bargains by anyone in jail.
"Anyone behind bars will do anything to get out, even lie," the congressman said.
Damous echoed Rousseff's criticism of frequent leaks of plea bargain testimony to Brazilian media before courts rule on whether the evidence is valid.
"Every time a defendant's plea bargain is leaked, it becomes truth the very next day," he said.
Additional reporting by Daniel Flynn; Editing by Kieran Murray