The attackers gained access to the house because two of the Spaniards were in the yard and apparently were forced to open the door, Walton said. The house is on a more isolated stretch of beach east of the city.
The victims were "psychologically affected" by the attack and received treatment, the mayor said. The lone Mexican woman in the group was not raped.
Guerrero state Attorney General Martha Garzon Guzman said witness descriptions of the attackers were more difficult to obtain because they wore masks.
Spain's Foreign Ministry had already issued a travelers advisory on its website for Acapulco before the Monday attack, listing the resort as one of Mexico's "risk zone," though not the worst.
"In Acapulco, organized crime gangs have carried out violence, though up to now that has not affected tourists or the areas they visit," the advisory states. "At any rate, heightened caution is advised."
The attack came just three days after a pair of Mexican tourists returning from a beach east of Acapulco were shot at and slightly wounded by members of a masked rural self-defense squad that has set up roadblocks in areas north of Acapulco, to defend their communities against drug gang violence.
The vigilantes say the Mexican tourists failed to stop at their improvised roadblock.
Walton said the city was already contemplating ways to revive the city's image.
"We have to look at an advertising campaign to say that not everything in Acapulco is like that," Walton said. "This happens everywhere in the world, not just in Acapulco or in Mexico."
Associated Press writer Bertha Ramos reported from Acapulco, and Mark Stevenson reported from Mexico City.