Dozens of bathers in a popular beach on the Parana River in the city of Rosario were left injured after a shoal of piranhas attacked them under water.
The injured, which included more than 20 children, saw a seven-year-old girl lose part of one of her fingers, while a young boy was left with an open fracture in his hand, the Daily Express reported.
Other swimmers, who were trying to cool down in 100 degrees Fahrenheit temperatures, suffered deep cuts in their ankles, fingers and hands as they raced out of the water bleeding from wounds and shouting for help.
The coastguards called paramedics so they could assist the most seriously injured before police temporarily closed off the beach, and forced people out of the water.
The attack, which has been blamed on a piranha cousin called palometas, was the most serious of its kind in the city since 2008.
Local government spokesman Ricardo Biasatti has described the attack as a rarity and said that this sort of event is absolutely sporadic in this area.