El Salvador's widespread violence turned an unexpected corner with the brutal and fatal beating of the National Zoological Park's beloved hippopotamus Gustavito.

Even among a population numbed by a staggering human death toll due to gang violence in recent years, the animal's death late on Sunday stirred outrage.

Salvadorans mourned through social media and some left flowers at the gate of the zoo, which has been closed until further notice.

"Here we're used to seeing the dead every day," Martin Castillo, a street vendor in the capital's historic downtown, said.

"They kill us like flies, but this tops it all. They killed an animal that only entertained us."

Most of the killings in El Salvador have been blamed on powerful street gangs. ( Reuters: Jose Cabezas )

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Zoo director Vladan Henriquez said at a news conference that the hippo was covered with bruises and puncture wounds after apparently being attacked with metal bars, knives and rocks.

He said the motive remains unknown.

The attack occurred last week, sometime on Tuesday night. But zookeepers did not discover the hippo's injuries until Thursday because he did not leave his pool.

Gustavito died from the injuries late on Sunday.

Justice Minister Mauricio Ramirez Landaverde said an investigation had been opened.

El Salvador is one of the world's most violent countries, recording 5,278 homicides last year, or an average of 14 people killed per day. Most of the killings have been blamed on powerful street gangs.

Gustavito was born and raised in Guatemala, but was brought to El Salvador 13 years ago.

The motive for killing Gustavito is still not known. ( Supplied: La Secretaría de Cultura de la Presidencia )

AP