Hundreds of supporters of the former president filled the streets in the southern city of Curitiba to protest Lula's innocence, saying he is the victim of a political plot.
Brazil's former leftist president Luiz Inacio Lula da Silva was set on Wednesday to give testimony in a court case that could put an end to his political career if he is convicted of the corruption charges he faces.
Lula, who leads opinion polls ahead of the 2018 presidential election, is accused of receiving a seaside apartment near Sao Paulo as a bribe from the OAS construction company.
The apartment and other alleged benefits from OAS are said to have been typical of a vast network of bribery uncovered by Operation "Car Wash," with major companies paying politicians to obtain influence with lawmakers and secure big deals with the Petrobras state oil company.
Senators, former ministers, and the once seemingly untouchable speaker of the lower house of Congress, Eduardo Cunha, have been arrested or convicted, while scores more high-flying politicians face probes.
Lula denies any wrongdoing, saying there is no proof he had anything to do with the apartment.
If he is found guilty, he will be barred from running for office again and could potentially face time in prison.
Lula became an icon of Latin America's left during a 2003-2010 presidency that saw Brazil enjoy a commodities-fuelled boom and tens of millions of people lifted out of severe poverty.
Now he is the country's most divisive figure, with opponents labelling him as the corruption kingpin and supporters saying he is the victim of a plot.
Protests against court hearings
Hundreds of people wearing the Workers' Party red colour T-shirts arrived in the southern city of Curitiba on buses from around the country to show their support for Lula.
"We came here because we understand that the trial against Lula is skewed, a prosecution that breaks Brazilian justice rules," said banking employee Jo Portilho, 54, after arriving in Curitiba.
Lula supporters have said they hope to bring as many as 30,000 people to demonstrate in the city this week. There are also expectations that his opponents will gather.