At least six people died when a two-story building under construction collapsed here Tuesday in Brazil's largest city, while 24 others were rescued alive from the wreckage, firefighters said.

First responders are looking for signs of life among the huge mass of cement blocks, broken beams and lumber, which Sao Paulo fire Capt. Marcos Palumbo described as "almost a labyrinth."

Firefighters maintained visual and verbal contact with two people still alive under the rubble, he said.

A 24-year-old man in the same situation called firefighters from his cell phone and guided them to where he was trapped under two concrete blocks.

His legs were pinned down by the concrete, but the firefighters managed to free him.

Palumbo said they recovered three bodies and have located another three buried under the building materials.

He noted that the number of fatalities could increase, since there are areas that have not yet been searched.

Some 35 people were working at the site when the structure collapsed at around 8:35 a.m., authorities said.

"They were working in a normal way, there was no noise, just a sudden collapse" of the structure, the fire department spokesman told the press.

The fire department sent 23 vehicles, 70 firefighters, two helicopters and dog teams to the scene of the disaster, while some 60 military police were also deployed there.

Authorities have not yet suggested a possible cause of the accident. EFE