The torching of a new playground in FDR Park drew a promise from Mayor Nutter to rebuild it and an $11,000 reward for information on the crime.
"This was an act of evil," Nutter said as he stood before melted plastic and charred fiberglass and metal.
The fire damaged about half the play area and two shade trees. The playground will remain closed until it is reconstructed.
It had just been completed at a cost of $306,000 in October, Nutter said; he called the site "a sacred place for children, for families, for any random individual to enjoy."
A Philadelphia Fire Department official said the fire was set. It was reported to the department about 9:25 p.m. Sunday.