Seven of the dead are men and two are women, officials said.

The chaos unfolded as supporters pushed through one of the access gates at the Monumental venue in Cuscatlan, around 25 miles northeast of the capital San Salvador.

It was hosting a match between clubs Alianza and FAS in the quarter-finals of the country's premier division.

Footage showed dozens of fans charge on to the field in what one first aid volunteer described as an "avalanche".

"It was an avalanche of fans who overran the gate," said the member of the Rescue Commandos group.

"Some were still under the metal in the tunnel.

"Others managed to make it to the stands and then to the field and were smothered."

Carlos Fuentes, spokesman for the Rescue Commandos, added: "We can confirm nine dead - seven men and two women - and we attended to more than 500 people.

"More than 100 were transported to hospitals, some of them were serious."

Some of those hurt received medical treatment on the pitch, with the match suspended after just 16 minutes.

In a statement, the Salvadoran Soccer Federation - the country's equivalent of the FA - said it regretted what had happened and voiced support for the victims' families.

Police said a criminal investigation would take place in partnership with the attorney general's office.