Time Out says
Though the beasts who participate in this 'fire run' have a long tradition in local festivals, the 'correfoc' itself was born in 1979 and started out as a Show of Fantastical Beasts that feature the participation mostly of Catalan groups from outside of Barcelona. The show, with roots in traditions such as the dance of the devils that dates back to the 14th century, was rechristened with the name 'correfoc' and became an immediate success that's since come to define La Mercè.
The beat of a traditional percussion group's drums signals the beginning of the correfoc, whose participants this year will begin their devilish trek from Plaça d'Antoni Maura. Be there for the lighting of the instruments that the devils and beasts carry, such as the 'ceptrots', pitchforks and banners; the Gate of Hell will also be lit, and so the correfoc begins. Revellers and hooligans alike then spread throughout the city setting off firecrackers and generally making a fiery ruckus.