A school of carnivorous fish attacked hundreds of Argentines cooling off in a river near the city of Rosario, sending at least 70 to local clinics and emergency rooms for treatment.
The attack by palometas, a type of piranha, occurred as city dwellers attempted to escape the 100-degree heat of Christmas Day in the Southern Hemisphere's summer season.
"There were some people that the fish literally had torn bits of flesh from," a medical official at the scene, Gustavo Centurion, was quoted as saying by the Latin Times newspaper.
Local lifeguards director Federico Cornier told the BBC and other broadcasters in the area that the attack was unusual in the number and ferocity of the bites.
"It's normal for there to be an isolated bite or injury, but the magnitude in this case was great... This is an exceptional event," Cornier said.
A 7-year-old girl lost a finger to the fish, and dozens of people suffered serious bites to their extremities, news agencies reported.
The incident occurred along the Parana River at Rosario, about 200 miles northwest of Buenos Aires.
Piranhas, a freshwater fish with sharp teeth, inhabit the rivers of South America. Several fatal attacks on humans have been reported in recent years, mostly in the Amazon River basin.