A huge school of piranhas attacked more than 70 Christmas Day swimmers seeking relief from the oppressive heat in a small town in Argentina.

At least 20 children were injured, including seven who lost parts of their fingers or toes during the horrific incident.

Witnesses said thousands of bathers took refuge from a 100-degree-plus heat wave by wading into the water of the Paraná River, near the city of Rosario, about 200 miles north of Buenos Aires.

Bathers suddenly complained of bite marks on their hands, ankles and feet, the city’s director of emergency services, Federico Cornier, said Thursday.

While some swimmers ran from the feeding frenzy, parents on the beach courageously charged into the water to rescue their children, according to witnesses.

Gustavo Centurion, a medical official, said the attack was “very aggressive.”

Gabriela Quintanilla, Argentina’s health undersecretary, told reporters, “There were some people that the fish literally had torn bits of flesh from.”

Despite the attack, many people went back to the water within half an hour because of the heat, paramedic Alberto Manino told the Todo Noticias TV channel.

Cornier blamed the attack on a huge swarm of palometas, “a type of piranha, big, voracious and with sharp teeth that can really bite.”

Attacks by flesh-eating piranhas are not unusual in South America.

When an 18-year-old man in Bolivia jumped out of a canoe earlier this month and was fatally bitten by piranha, police suspected it was a suicide because he was a fisherman who would have been aware of the danger. But the extent of Wednesday’s attack was rare, officials said.

“This is not normal,” Cornier said in a television interview. “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.”

Some officials said the unusually warm weather was apparently responsible for the fish congregating near the river’s surface before the attack.

Others said bait or debris left by local fishermen may have lured the large number of palometas to the area.

Police had to force some bathers out of the water after the attack, and the beach was closed.

Officials in Rosario, known as “Argentina’s Chicago,” said it was the most serious palometa attack there since 2008, when 40 swimmers were hurt.

•Credit (except headline): The New York Post.