Five adults and three juveniles were shooting up the Los Anonos section of Escazú early Sunday.
The celebration drew the attention of police, who collared the eight and confiscated an arsenal.
In addition to three riot guns. police reported they confiscated a revolver and an AK-47 rifle with banana clips.
As police entered homes to make the arrests, they also discovered some 64 baggies of marijuana, they said.
Police were unable to determine the nature of the celebration that generated the shots into the air.