Lula, 71, is accused of receiving a seaside apartment as a bribe in a much wider corruption scheme investigated by the so-called "Car Wash" probe upending Brazilian politics.
He says the charges are trumped up to stop him seeking reelection in 2018. His passionate supporters gathering in the southern city of Curitiba agree.
Wearing the Workers' Party red color and waving red flags, hundreds of people came in on scores of buses from around the country to back Lula ahead of his hearing with senior "Car Wash" Judge Sergio Moro.
There were high-profile allies of the beleaguered former president too. Workers' Party Senator Humberto Costa flew in Tuesday and Lula's successor in the presidency, Dilma Rousseff, was due in on Wednesday, a spokesman said.
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Lula, 71, became an icon of Latin America's left during a 2003-2010 presidency that saw Brazil enjoy a commodities- fueled boom and tens of millions of people lifted out of severe poverty.
The fiery, bearded orator who rose from great hardship to found the Workers' Party and lead Brazil, is already one of the most famous people in the country.
Moro, a quiet, intense 44-year-old legal high-flier, is rapidly catching up. As the face of the "Car Wash" probe that has gripped Brazil for more than two years, Moro is a hero to many.
Now, Moro is weighing allegations that Lula accepted a luxury seaside apartment near Sao Paulo as a bribe from the OAS construction company.
Already, 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.