El Salvador was in shock on Sunday after 12 people died and hundreds were injured in a stampede at a soccer stadium, as the country's president vowed an investigation.

Authorities said initial reports pointed to a crush of fans who tried to enter the 35,000-seat Cuscatlan Stadium in the Central American country's capital San Salvador to watch a game between two local teams, Alianza and FAS.

The match was suspended as emergency personnel evacuated people from the stadium, where hundreds of police officers and soldiers gathered as ambulance sirens wailed.

Carlos Fuentes, spokesman for the emergency services group Comandos de Salvamento, said they were treating more than 500 people for various injuries, while civil protection authorities said 88 people in total where hospitalized.

The stampede apparently started after a stadium gate fell, causing people to crowd together, Fuentes said.

Fredy Alexander Ruiz, a 28-year-old survivor, said he was "traumatized from seeing people thrown on the ground, dead, bruised, with their faces stepped on."

The stampede started 10 minutes into the game and after it was suspended even the players joined in the frantic rescue efforts.

"I had five people on top of me that were suffocating me," said Ruiz. "Thank God, I was able to grab the foot of a policeman, and he and a friend of mine pulled me out."

El Salvador's President Nayib Bukele said authorities would investigate the incident and those responsible would be punished.

"Everyone will be investigated: teams, managers, stadium, box office, league, federation," Bukele said on Twitter.

He warned that "whoever the culprits are, they will not go unpunished."

The Salvadoran Football Federation (Fesfut) said in a statement it "deeply regrets" the events that occurred at the stadium and "expresses its solidarity" with the families of those "affected and killed."

"Fesfut will immediately request a report of what happened and will communicate the relevant information as soon as possible," it said. (AFP)