No one was hurt in Saturday's blaze, after firefighters speedily extinguished flames at the Tomorrowland festival in Santa Coloma de Gramenet in the country's northeast.
But around 20 people had to be treated for anxiety attacks or minor injuries, the fire service said in a statement, noting that 22,143 people had to be removed because of the inferno.
Footage of the incident showed one side of the stage consumed by giant flames, a shower of sparks raining down as people ran away and black smoke billowed up.
It was not clear exactly what caused the fire but organisers said on their Facebook page it was due to a technical malfunction.
"Thanks to the professional intervention of the authorities all 22,000 visitors were evacuated safely and without reports of injuries," they said.
According to other images on social media the concert appeared to be drawing to a close after a pyrotechnics show when the fire started.
"The safety plan for the event worked perfectly," said the official, noting that there had not been panic among the crowd.
Video showed a giant screen displaying the message: "Remain calm and follow instructions."
Fire experts were due Sunday to investigate at the scene of the blaze, alongside officials from the company that erected the stage, having first removed any dangerous remnants.
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