There were no reports of any fatalities from the incident which the Galicia regional government said left 316 people injured at the O Marisquino festival in the town of Vigo.
Nine of them are still in hospital and two are in the intensive care unit but their lives are not in danger, city hall said in a statement.
Vigo’s mayor Abel Caballero said the platform that collapsed just before midnight was 30 meters long and 10 meters wide, Reuters reported.
City hall added that Pedro Saura, junior minister for public works, has ordered an investigation into the cause of the collapse.
People attending the concert said the crowd was enjoying the concert when they heard the boardwalk breaking.
“We were jumping when we heard the sound and we saw everything collapsing,” one woman told state broadcaster RTVE.