Former goalkeeper Leao, who played for Brazil at the 1974 and 1978 World Cups and coached his country from 2000-01, had an unhappy second spell in charge at Sao Paulo who have made a dismal start to the Brazilian championship.
Sao Paulo have taken nine points from their opening six matches with Saturday's 1-0 defeat by promoted Portuguesa the catalyst for Leao's dismissal after nine months as their coach.
"I want to inform you that I'm no longer Sao Paulo's trainer," Leao said.
Juvenal said assistant coach Milton Cruz would take charge of the team as a caretaker.
"Brazil doesn't shine with its coaches, we have brilliant players but not coaches," he said, adding he had negotiated without success to take on a foreign trainer.
Sports websites reported that Sao Paulo had approached former Chelsea manager Andre Villas-Boas but the Portuguese had turned them down.
Leao's sacking was the second of the championship, which began in mid-May, after Adilson Batista lasted a mere two months and two matches, both draws, at Atletico Goianiense.
This year there have already been 12 changes of coaches among the 20 teams in the Brazilian first division, the other 10 coming during the state tournaments that precede the national championship.
The results based culture in the Brazilian game produced 33 changes in 2011.