Fire Commissioner Lloyd Ayers says dozens of paramedics and fire crews are on the scene working to find those trapped. WPVI-TV reports that at least five people have been taken to hospitals.
The collapse involved a Salvation Army corner thrift store and a four-story building next door with a sandwich shop on the first floor. It's not clear whether any other adjacent buildings were damaged.
There is no word on exactly what caused the collapse.
The accident happened at 10:45 a.m. Wednesday on the western edge of downtown, between the city's business district and its main train station. A building on the block has been under construction.