He is accused of receiving a seaside apartment as a bribe in a much wider corruption scheme investigated in the so-called "Car Wash" operation that has upended Brazilian politics.
Wearing the Workers' Party red and waving red flags, hundreds of people came in on buses from around the vast country to back Lula, who will face senior "Car Wash" Judge Sergio Moro on Wednesday.
The leftist movement Free Brazil Front told AFP that 100 buses of activists had already left Sao Paulo for Curitiba.
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.
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.
But now facing a total of five corruption cases, he is fighting for his reputation and the chance for an epic comeback in elections next year.
Although heading opinion polls, Lula has become the country's most divisive figure, with opponents painting him as the corruption kingpin and supporters calling him the victim of a plot.
"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, just after arriving in Curitiba.
The crowds were there to show Moro that "the people are watching, observing," he said.
Clash of personalities
Lula, 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.
He is weighing allegations that Lula accepted a luxury 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 "Car Wash," with major companies paying politicians to obtain influence with lawmakers and secure big deals with the Petrobras state oil 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.
Lula denies any wrongdoing, saying there is no proof he had anything to do with the apartment. If he is found guilty when Moro hands down his verdict in an estimated 45 to 60 days, and then loses an appeal, he'd be barred from running for office again and face prison.
Plea for calm
Lula supporters have said they hope to bring as many as 30,000 people to demonstrate in Curitiba this week. There are also expectations that opponents will gather.
Fearful of violence, the authorities are pleading for people to stay away and are stepping up security.
"The idea is to make sure there is no contact between the two groups," said Curitiba security chief Wagner Mesquita.
On Saturday, Moro took the unusual step of issuing an appeal on Facebook.
"We want to avoid any kind of disturbance or confrontation. I don't want anyone hurt. Don't come, it's not necessary. Let justice take its course," he said.
Ever the fighter, Lula appears to be looking forward to his day in court.
"It's been two years that the media have been saying I'll end up in prison. Well, they should arrest me soon or otherwise it could be me having them arrested due to all their lies," he said Friday at a Workers' Party event.
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