At least 60 people were injured in the Argentine city of Rosario when they were attacked by a swarm of piranha fish, officials said.

The attack happened off the popular beaches of the Parana River, 300km north of Buenos Aires, on Wednesday, a medical official said on Thursday.

"This is not normal," said Federico Cornier, the director of emergency services in the city of Rosario. "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."

A seven-year-old girl had her finger partially amputated and dozens more suffered bite wounds on their extremities from a fish called "palometas," a relative of the piranha, said Cornier.

Vicious attack 

Health officials called the attack "very aggressive".

"There were some people that the fish literally had torn bits of flesh from," Gabriela Quintanilla, health undersecretary, told reporters.

Temperatures in the region soared to about 38 degrees Celsius, prompting thousands to seek relief in the Parana River, which is packed with carnivorous fish.

Officials said the unusually warm weather was partly to blame for the the fish congregating on the river's surface.