Last year, Moro convicted da Silva of trading favours with a construction company in exchange for the promise of a beachfront apartment.
That conviction was upheld by an appeals court in January.
He knows what it is like to have a tough life and loves the poor more than the rich," said Antonio Ferreira dos Santos, a 43-year-old bricklayer who was keeping vigil outside the union.
Workers' Party leaders insist that da Silva, 72, would still be the party's candidate in October.
Born in the hardscrabble northeast, da Silva rose through the ranks of the union in the country's industrial south." said former president Dilma Rousseff, who succeeded da Silva and in 2016 was impeached and ousted from office."They fear that Lula would get a favourable decision in [a higher appeals] court.Moro's warrant on Thursday came after Brazil's top court, the Supreme Federal Tribunal, voted 6-5 to deny a request by the former president to stay out of prison while he appealed a conviction that he contends was simply a way to keep him off the ballot in October's election.He is the front-running presidential candidate despite his conviction.