El Salvador's widespread violence has reached an unexpected corner with the fatal beating of the national zoo's beloved hippopotamus Gustavito.

Even among a population numbed by a huge human death toll due to gang violence in recent years, the hippo's death caused outrage.

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

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

"Here we're used to seeing the dead every day," said Martin Castillo, a street vendor in central San Salvador. "They kill us like flies, but this tops it all. They killed an animal that only entertained us."

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.

The attack occurred last week, but zookeepers did not discover the hippo's injuries until Thursday because he did not leave his pool. Gustavito died of his 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.

"We're angry," said Carmen Rogel, who often takes her grandson to the zoo. "We didn't know they had killed Gustavito and were surprised when we arrived and the gate was closed."