Brazilian President Dilma Rousseff has named her predecessor Luiz Inácio Lula da Silva as chief of staff.

Lula da Silva's appointment prompted protests in Brazilian cities. He now has legal immunity in a corruption investigation -- known as Operation Car Wash -- involving state-run oil company Petrobras.

According to police, 2,500 people demonstrated outside the presidential palace in the capital of Brasilia and São Paulo's main Avenue Paulista. A session of Congress was also disturbed by dozens of opposition lawmakers, who chanted for Rousseff to step down from her position.

Rousseff, Lula Trying to Influence Prosecution

Sergio Moro, the federal judge overseeing the corruption investigation, said in a court filing that the taped phone conversation showed Rousseff and Lula da Silva attempting to manipulate the prosecution so he can be exempted.

"I observe that, in some dialogues they talk about, apparently, trying to influence or obtain assistance from prosecutors or the courts in favor of the former president," Moro wrote in the filing issued on the court's website on Wednesday, as reported by Reuters.

Rousseff, meanwhile, thanked Lula da Silva in a press conference for helping her address the issues currently pervading her administration.

"He comes with his knowledge of the country, of the needs of the country and his commitment ... it is going to be a huge boost for my government," the president said, as quoted by CNN. "He comes with his political capital; he's a great communicator. He makes me very comfortable."

Lula da Silva's supporters hope that he can prevent Rousseff from impeachment.

Rousseff insisted that Lula da Silva was appointed for his political experience and his history of improving fiscal stability and fighting inflation. She noted that despite being chief of staff, Lula da Silva will still be subjected to investigation by Brazil's top court.

State prosecutors in São Paulo charged him with money laundering and fraud and asked for his preventive detention. However, Brazil's law dictates senior political officials can only face trial in the Supreme Federal Tribunal. Lula da Silva could be tried at the SFT, but the process would be long.

Phone Recording a Sign of Connivance?

In the phone recording obtained for the corruption investigation, Rousseff was heard saying she would send a document confirming Lula da Silva's chief of staff post so he could use it "if he needed," CNN reported.

Critics said that this conversation is a political scheme to protect Lula da Silva. On Wednesday, Rousseff dismissed claims that his appointment would oust Central Bank President Alexandre Tombini or push her administration to use its reserves.