The incident occurred during a match between local team Alianza and the Santa Ana-based team Fas at Cuscatlán Stadium.
The match was subsequently suspended.
President Nayib Bukele described the event as "unprecedented", and said an "exhaustive investigation" would be launched.
Police had earlier said that seven men and two women were among the dead, with all the victims over the age of 18.
The disaster reportedly occurred after a large number of fans tried to enter the venue after the gates had been closed.
