It happened at 49th Street and Sixth Avenue shortly before 11:30 a.m.
Witnesses say the taxi driver jumped the curb and hit her.
Several bystanders ran to her assistance before first responders arrived on the scene.
Police say doctors at Bellevue Hospital are working to reattach her leg which was severed below the knee.
"I see the yellow cab on the sidewalk and I see her without her leg and her and cut. I do my best I collect all the ice from all the vendors," said one witness.
The cab driver was not injured.
There is no word yet on any charges.
A biker also suffered minor injuries from the accident.
[NY Daily News Report]