"Happily, the majority got out," de Souza told local reporters.
Flabio Gabia, a receptionist at a nearby hotel, told Estadao newspaper: "When I went to see what it was, the streets, which were deserted, were filled with desperate people."
De Souza and other residents speculated to media that the fire was caused by a gas leak.
Brazilian President Michel Temer visited the ruins early Tuesday morning, but rushed away after victims began throwing rocks and debris at his car. "I had to come, despite these protests, because I was in Sao Paulo, and it would have looked bad if I didn't," he told journalists. Victims shouted at him to "leave".
The fire highlights the perils of Sao Paulo's affordable housing crisis. Highly organised squatters have taken over as many as 70 buildings in the downtown area of South America's largest city. The government estimates that nearly 4000 families are living in vacant buildings downtown. According to Associated Press, "many such dwellings are run like regular apartment buildings, with doormen and residents paying monthly fees and utility bills. Others are less established and more precarious."
Former Sao Paulo Mayor Joao Doria aggressively cracked down on these communities.
He had envisioned a revitalised downtown, a glitzy showcase for an important financial centres. Fair-housing activists had criticised his efforts saying he should focus on creating affordable housing to the tens of thousands who need it.
Officials say it will take days to clear the fire debris.