Authorities are investigating the cause of a three-alarm blaze that damaged two Center City apartment buildings and a restaurant under construction.

Fire officials said the blaze at 13th and Chancellor streets was reported shortly before 4:30 a.m. Tuesday.

Crews encountered heavy fire conditions, bringing the blaze to three alarms and prompting 100 firefighters to respond to the scene.

The six-story Gramercy apartment complex and the 24-story Chancellor building were both evacuated.

"That's where our biggest challenge was, getting all those people out safely," Fire Commissioner Derrick Sawyer said.

The blaze was believed to have started at a permitted construction site, which was previously the razed Letto Deli.

The new building is to house Maison 208, a French restaurant to be run by chef Sylva Senat.

It took a little more than an hour for crews to get the blaze under control, though firefighters continued to douse hot spots.

"They did a great job of knocking down the fire and addressing the heat conditions," Sawyer said.

As crews worked to put out the hot spots, police tape surrounded the damaged construction fencing and toppled wood beams.

It wasn't known how or exactly where the fire started. The fire marshal's office will investigate the cause of the blaze.

No residents or firefighters were injured, the fire department said.

Some residents said they initially thought the blaze sounded like a gunshot or fireworks.

"I didn't know if it was a real fire, but I worke up and got startled," said 26-year-old Christian Vellani, who has lived there for three months.

Late Tuesday morning, he said he had been outside since 4:30 a.m.: "I don't have any of my stuff, no phone, no wallet."

Zac Pham, who was visiting a friend, said he heard two booms and saw smoke in the third-floor hallway.

"I was right near where everything was collapsing," he said. "A big fireball just went boom. Debris fell in the streets."

Sawyer said he hoped residents in the Chancellor would be able to return to their homes later Tuesday, but it wasn't known when people would be able to get back inside the more heavily damaged Gramercy. The Red Cross and Office of Emergency Management were assisting displaced residents.

Streets around the blaze were closed and several SEPTA bus routes faced detours for several hours Tuesday morning. By about 7:30 a.m., only Route 12 was still being detoured.

Emily Babay is a Philly.com staff writer. Dani Blum is an Inquirer staff writer. Philly.com staff writer Michael Klein contributed to this article.