The platform had been packed with people watching a rap artist at the two-day O Marisquiño event in Galicia.
According to eyewitnesses, the wooden platform gave way during the first song of the concert as Rels B, who had just started his performance, told the crowd to jump, local media report.
Police confirmed that there were no known fatalities.
Witnesses reported scenes of panic as people – many of them teenagers – tried to scramble to safety. Some fell into the sea.
Personal effects such as mobile phones and handbags were left scattered on the ground.
Several emergency teams attended to the injured, and divers were sent into the sea to check that no-one was trapped beneath the structure.
The incident happened shortly before midnight on August 12 as a crowd was listening to the Majorcan rap artist Rels B.
Earlier, the regional health minister, Jesus Vazquez Almuina, told local radio about the nature of the injuries people suffered.
He said: “These are provisional figures, patients are still being evaluated… The vast majority are light injuries for bruises. There are five hospitalizations, mainly broken bones and some head injuries.”
In a tweet, Rels B wished “strength” to the injured and advised anyone worried about friends or relatives to contact an emergency information point set up by festival organizers.
The mayor of Vigo, Abel Caballero, has said there will be an investigation into the causes of the incident.
It is not yet clear whether the platform collapsed because there were too many people on it, or because the structure itself was weak, or whether other factors were involved.
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