Emergency crews responded to a wreck at the Huntsville Veterans Memorial about 45 minutes before the Veterans Day Parade was scheduled to begin.
Witnesses on the scene said a woman had a medical emergency, lost control of her van and flipped at the intersection of Monroe Street and Jefferson Street.
Police said when the van overturned, it struck three spectators.
Emergency workers said four people were transported to the hospital from the scene.
The driver, a 52-year-old female, was taken to the hospital. Her 17-year-old daughter was treated for her injuries at the scene.
The van struck a 40-year-old female spectator, her 63-year-old mother and her 8-year-old son. They were also taken to the hospital to be treated for injuries.
All four of the victims transferred to the hospital suffered non-life threatening injuries.
Investigators said it doesn't appear that the driver of the van was speeding. The cause of the accident is under investigation.
Police cleared the scene and the parade started at 11 a.m. as planned.