The gas leak happened Saturday at the church of Sant Vicenc de Gualba during a concert that drew a crowd of about 130 people, ThinkSpain reported.
Of the 35 affected, three were in a serious condition -- an 81-year-old woman, a 50-year-old woman and an 8-year-old boy -- and the rest suffered only mild effects.
Officials said the gas leak was caused by "faulty combustion of the heaters used during the concert."
The church was ventilated overnight and firefighters said Sunday the levels of carbon monoxide were back down to normal and it could be reopened to the public.
Swim Week Miami Beach 2014 [PHOTOS]