It took more than four hours to bring the blaze under control as firefighters battled to keep it from spreading to other structures on a block filled with shops, small businesses and apartment houses.
The fire, which started with a single alarm at 5:02 a.m., had reached six alarms by 6:47 a.m.
"We thought it was better to be safe than sorry," said Philadelphia Fire Commissioner Harold Hairston.
By the time officials declared the blaze under control at 9:15 a.m., the once-stately brownstone - which for 30 years has housed the Metropolitan Reporting Bureau, a firm that compiles records for insurance companies - had been reduced to a smoking hulk.
Hairston said the scores of insurance records kept on the property probably fed the flames.
"It's pretty obvious there was a tremendous fire load," he said.
There were no injuries, and the cause of the blaze remains unknown. The investigation was continuing.
To vent heat and smoke, firefighters shattered the windows of an architectural office in the building at 1915 Walnut, and propped open the doors of the Rittenhouse Eric Theater at 1911 Walnut.
Both buildings sustained severe water and smoke damage.
The Eric remained closed yesterday as fire crews spent much of the afternoon pumping water from the multiplex theater and other buildings on the block.
Around 150 workers were displaced by the blaze, and many stores along the street remained closed for all or part of yesterday.
"The holidays are not exactly the optimum time for a disaster," said Sandra Safron, owner of Born Yesterday, a children's specialty shop at 1901 Walnut St.
Fire officials said the entire 1900 block of Walnut Street might have been heavily damaged had it not been for a worker dozing in the architectural offices of Stuart G. Rosenberg at 1915 Walnut early yesterday.
Awakened by the smell of smoke, he made the only 911 call to report the fire.
Firefighters received that call at 5:02 a.m., according to Battalion Chief Charles Lindsay, a spokesman for the Fire Department. Engine companies from Battalion 1 in Center City were the first to respond, arriving within two minutes, Lindsay said.
Entering the building, firefighters found thick smoke and searing heat. Unable to pinpoint the blaze, which officials now believe started on the second floor, they withdrew after 12 minutes, Lindsay said, and began what would be an hours-long process of pumping water into the building. Hairston said he believed the flames raced inside the walls before breaking free, causing the collapse of the second and third floors as well as a large section of the back wall and the rear segment of the roof.
From her apartment on the 15th floor of the Wellington building at 135 S. 19th St., Doris Stroud could see fire and smoke rushing through a gaping hole in the roof.
"My daughter looked out first, and she thought it was snowing, there was so much smoke," said Stroud. "When I looked out, I could see flames coming up from a very stubborn spot where the fire would die down for a while, then shoot back up."
The loss of the brownstone is a blow to Rittenhouse Square's considerable legacy of historic structures.
It was built in 1897 by architect Joseph M. Huston and served as the home of Francis L. Potts, scion of a successful iron mining family.
The building is listed by the Philadelphia Historical Commission, whose records describe it as "an imposing . . . brick townhouse with granite columned porticos." The value at the time of its construction was around $75,000, according to Historical Commission records.