The city of Philadelphia thinks you need a permit...
REGULATION GOVERNING PERMITS FOR DEMONSTRATIONSON CITY PROPERTY
Section 2. No person or entity shall conduct or hold a Demonstration on City Property without firstobtaining a Permit from the City
Well, there's a difference between speaking and holding a demonstration. A city needs to be able to plan for that type of thing. What would be unconstitutional is for the city to deny that permit if you weren't going to be impeding on anyone else's rights.