BRASILIA/SAO PAULO: Protests erupted in Brazilian cities on Wednesday after President Dilma Rousseff named her predecessor Luiz Inacio Lula da Silva chief of staff and a taped telephone conversation fed opposition claims the appointment was meant to shield Lula from prosecution.

Police said 2,500 people demonstrated outside the presidential palace in the capital of Brasilia and others flocked to Sao Paulo's main Avenue Paulista. Dozens of opposition lawmakers interrupted a session of Congress, chanting for Rousseff to resign.

Lula has immunity from all but the Supreme Court after his nomination as Rousseff's chief of staff was published in a special edition of the Official Gazette. State prosecutors had charged him with money laundering and fraud, and asked for his arrest.

Federal Judge Sergio Moro, who oversees the sprawling corruption investigation, said in a court filing released on Wednesday that the phone conversation showed Lula and Rousseff considered trying to influence his enquiry.

"I observe that, in some dialogues they talk about, apparently, trying to influence or obtain assistance from prosecutors or the courts in favour of the former president," Moro wrote in the filing published on the court's website.

Moro said there was no information that any attempt to influence authorities was actually carried out. The recording, made public by the court, also has Rousseff offering to send Lula a copy of his appointment, "in case it was necessary."

Rousseff's opponents have called the appointment of Lula, a charismatic champion of the poor who remains one of Brazil's most influential figures six years after leaving office, was a desperate attempt to stave off ongoing impeachment proceedings and to spare the former president from arrest.

Rousseff said Lula was appointed for his experience and had a history of championing fiscal stability and combating inflation. She said his appointment did not mean he is above investigation as he could be tried by the country's top court.

Lula, Rousseff and her ministers have denied any wrongdoing. Lula's lawyer warned the release of the recording could result in a "social convulsion."

Opponents warned that Lula, who has called for more public spending to end Brazil's worst recession in decades, may push Rousseff to abandon the government's austerity measures.

The graft probe, named for a money laundering investigation that started at a car wash in the capital Brasilia, has rattled the heights of Brazil's political establishment and jailed dozens of prominent business leaders.

Impeachment efforts focused originally on accusations that Rousseff intentionally broke budget rules to boost government spending during her 2014 re-election campaign, but gained steam as corruption allegations reached her inner circle.