Pictures posted on social media showed festival goers running away from a stage engulfed by flames, Xinhua news agency quoted Sky News as saying.
The Director General of Civil Protection in Catalunya posted on Twitter that no injuries had yet been reported.
Pyrotechnics on the stage have been blamed, British newspaper Daily Mirror quoted Spanish media as reporting.
A video shared on social media shows how the blaze erupted in a matter of seconds as the speaker system was engulfed in fire, triggering music fans to run for safety.
The emergency evacuation procedure flashed on the stage screen as the flames erupted, according to Daily Mirror.
(Only the headline and picture of this report may have been reworked by the Business Standard staff; the rest of the content is auto-generated from a syndicated feed.)