MADRID, Spain (AP) - Spanish authorities have ordered a section of Madrid's business district closed Monday following an intense fire that heavily damaged one of the city's tallest skyscapers, leaving it still standing but unstable.

The fire, which left seven people slightly injured, broke out in the 32-story Windsor building on Saturday at about 11:20 p.m. Thick smoke and searing temperatures prevented firefighters from entering the building and getting the fire under control until late Sunday.

The office tower was heavily damaged but did not collapse, as had been feared. However, officials said it was unstable and closed the area around the building.

"What worries us now is its structural state because of the high temperatures it was subjected to," Merardo Tudelo, director of the Madrid Municipal Firefighters, told reporters.

Madrid Mayor Alberto Ruiz-Gallardon said "the situation is still critical."

Emergency officials planned to keep the area in the Spanish capital's banking and business district cordoned off at least through Monday _ a closure that would affect dozens of businesses and several thousand employees.

The department store El Corte Ingles, located alongside the wrecked skyscraper, confirmed it won't open Monday and told its 2,000 employees to stay home.

Cars will be routed to neighboring streets, subway lines under or near the damaged Windsor Building will remain shut down, and adjacent office towers will remain closed by order of the mayor.

"This is the biggest fire ... this city has ever had," Gallardon told reporters outside the blackened hulk of twisted wreckage at midday.

It was not immediately clear what caused the fire, but the building was almost empty when the first alarm went off. Only one of the seven firefighters who suffered smoke inhalation remained hospitalized Sunday, Gallardon said.

By Sunday evening, flames were no longer visible, though gray smoke and ash stoked by gusts of wind continued to pour from the blackened shell of the building.

Hours earlier, several top floors collapsed onto lower ones. Firefighter official Fernando Munilla expressed concern that the entire building _ which at about 350 feet high is among the 10 tallest in Madrid _ could collapse.

"If the partial collapses keep happening, it would be lying to say it's impossible that the whole building couldn't fall down," he said.

Construction of the shiny gold Windsor Building began in 1973 and was completed in 1979. It became a landmark structure in Madrid's business district. The building was surrounded with scaffolding due to recent repairs, and a huge crane remained perched on its roof.

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Associated Press writer Mar Roman contributed to this report.