Fire crews responded just after 1:30 a.m. to find part of the structure engulfed in flames.
The flames grew so high they even licked the side of the building that also has an apartment building above.
It took about 20 minutes to extinguish the blaze.
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Sources who viewed surveillance video say it appears homeless people were using the outdoor "streetery" and set a fire. They were reportedly seen running out after the fire started.
Residents say at night, the outdoor dining structures often have homeless seeking shelter. The "streeteries" became popular during the COVID-19 pandemic.
"It's really scary, nerve-racking. It could happen at any time," said Samantha Schwartz of Rittenhouse Square.
The fire did not damage the actual restaurant inside, which continued with dining service as normal on Friday night.
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The fire marshal's office is investigating.
Anyone with information is asked to call Philadelphia police at 215-686-TIPS.