She appeared with Lula on the balcony of his apartment and waved to hundreds of people who had gathered below.
Lula has said his brief arrest on Friday is part of a campaign to sully his image and that of Ms Rousseff.
Police are looking into payments and donations made to Lula's institute.
Some of Brazil's wealthiest people as well as dozens of politicians from both the governing coalition and the opposition are also being investigated for involvement in the alleged Petrobras corruption scheme.
Lula, a left-wing icon, left office in 2011. His Workers' Party has been hit hard by the long-running scandal.
After his interrogation on Friday, he told reporters he was the victim of a "prejudice as a working-class man".
Ms Rousseff turned up at his home on Saturday, along with hundreds of people showing support for the former president.
Saturday's rally was peaceful in contrast to angry scenes on Friday when protesters clashed with police outside the building.
"She is going to meet with Lula as a gesture of solidarity and support," a press officer at the presidential palace told the Associated Press news agency.
She later could be seen on the balcony with Lula and his wife Marisa.
The Workers' Party has held the Brazilian presidency since 2003, both under Lula and Ms Rousseff.
In the latest operations, police enforced 33 search and 11 detention warrants in the states of Rio de Janeiro, Sao Paulo and Bahia, officials said.
Lula, 70, is suspected of receiving about 30m reais ($8m; Â£5.6m) in speaking fees and donations to his charity.
His home was among the premises targeted, as was the headquarters of the institute in Sao Paulo.
Brazilian President Dilma Rousseff has visited her predecessor, Luiz Inacio Lula da Silva, a day after he was questioned over corruption allegations at the state oil company, Petrobras.