A total of 23 million gallons of water was lost, said Laura Copeland, spokeswoman for the Philadelphia Water Department.
Repairs will take months, she said.
"There's a lot of work to be done," Mayor Nutter said.
The mayor praised firefighters and Water Department workers for their quick response and handling of the main break, which flooded the intersection under as much as five feet of water.
Water pressure was eventually restored to thousands of homes and businesses.
The intersection will be closed while the repair work continues, Copeland said.
"We recommend motorists avoid that area," she said.
No injuries were reported, but officials evacuated about 35 children from a day-care center at the intersection and 300 students from a nearby charter school.
Madeline McClain, supervisor at Nana's Daycare, was sitting at her desk near the front door when she heard a loud crash just before 9 a.m.
She looked up and saw water rushing into the parking lot. She alerted her staff that they needed to get the children and evacuate.
With the help of police and firefighters, they moved the children to a school on the next block.
"We put a lot of the babies in strollers" and rushed everyone to safety, McClain recalled from the day-care center's sister location at H and Cayuga Streets.
Six residents were relocated but were allowed to return to their homes by early afternoon, Nutter said at a news conference at the sodden intersection.
Because of low water pressure, 44 public schools in surrounding areas closed early.
A Water Department computer monitor detected a sudden loss of water about 8:50 a.m., and workers quickly pinpointed the location, Water Commissioner Howard Neukrug said.
The department notified residents and businesses in eight zip codes about the break and the possible impact on their water service.
Nutter and Neukrug said it was too early to determine the cause of the break of the 48-inch main.
Neukrug said that because of the main's depth, freezing and thawing did not appear to be a factor. He also said cast iron pipes of that age - this one dates to 1907 - are not necessarily likely to fail.
Firefighters first on the scene smelled gasoline and were concerned that the rushing water had caused fuel from underground gasoline tanks to spill into Frankford Creek.
Officials later determined that there was no major leak and that the gas probably came from the tanks of flooded cars.
City officials said 19 locations, all businesses, had reported that they were without power because of flooding.
The neighborhoods affected by the break include Frankford, North Philadelphia, Kensington, Port Richmond, and Juniata Park.
Neukrug said the 48-inch main feeds off a 60-inch main that was built in 1906.
"It's a major pipe that provides water for large parts of the city," he said.
Businesses in the area closed after the break. At a neighborhood Walgreen store, employees wrapped in blankets looked out the front door with the lights off and a sign that read, "Closed."
A nearby 7-Eleven store was open but the bathroom was closed, and a handwritten sign alerted customers that a McDonald's was closed for the time being.
A Dunkin' Donuts on the opposite end of the block was open and doing business. Its water line is connected to a different main.
Employees at the two car dealerships said they hoped the city would reimburse businesses.
But they could run into a situation similar to the one that resulted from a large water-main break at 21st and Bainbridge Streets in 2012.
The claims for damage to the mostly residential area were nearly $3 million. Yet under a cap set by state law, the city is obligated to pay only $500,000 total to all affected parties.
Inquirer staff writer Robert Moran contributed to this article.