More than 150 firefighters are battling the blaze.
The blaze is now under control, the spokesman added.
Witnesses said flames spread quickly from one of the lower floors and set an adjacent building on fire.
A rescue worker along with a sniffer dog searches for victims on a collapsed building in downtown Sao Paulo, Brazil May 1, 2018.
The cause of the fire is now unknown, according to the spokesman, who added that he does not know the total number of floors in the building due to the partial collapse.
Footage of the building collapsing has been shared online, showing huge flames and billowing black smoke. Then squatters moved in; an Associated Press investigation previous year found about 350 families living there.
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Speaking to the BBC , Mr Coupe said divestments were possible, but there were no plans to close shops and customers would be unaffected.
The city of Sao Paulo is Brazil's most populous, and it is among the world's most populous cities.
There have been unconfirmed reports that there were people on the top floors when the building collapsed.
A neighbouring building was evacuated, shortly before it caught fire.
Brazilian President Michel Temer visited the ruins early Tuesday morning, but rushed away after victims began throwing rocks and debris at his vehicle.
Then-mayor of Sao Paulo, Joao Doria, had launched an aggressive crackdown on these communities. Fair-housing activists had criticised his efforts saying he should focus on creating affordable housing to the tens of thousands who need it.
Firefighters believe clearing the rubble, which was still smouldering hours afterward, could take days.