The accident happened around 1:00 p.m. at 5th and Chestnut streets in Center City.
Initial reports indicate man was working on the Lafayette Building when a rock fell on his leg.
Medics rushed the 47-year-old to Jefferson University Hospital.
Those medics say the man's leg was 90 percent amputated.
The man's name has not been released.
Find out about Breaking News as it happens. Sign up for 6abc.com Breaking News Alerts.