But local daily El Faro de Vigo said it happened shortly after Spanish rap artist Rels B told the crowd to jump at the start of his concert.
Five people are thought to be seriously injured.
"There were loads, I mean loads of people" on the boardwalk, one witness told RTVE. Five are in a serious condition, and two remain in intensive care with serious - but non-life threatening - injuries.
The president of the Vigo port authority, Enrique Cesar Lopez Veiga, said he suspects there was a structural problem in the boardwalk, which is supported by concrete pillars. "People screamed, shouted. I asked for help to find people who fell down into the hole by the sea, including a friend of mine, who is in the emergency room".
It is unknown whether the boardwalk collapsed due to there being too many people on it, or if the structure itself was weak.
Police, rescue and emergency crews worked for hours to get people out of the water, set up a triage site and send the injured to nearby hospitals. "I got my foot stuck". I tried to get out and couldn't.
He added that Vigo's junior minister for public works, Pedro Saura, has ordered an investigation into the cause of the collapse. The free music festival was also giving spectators the chance to participate in extreme sports.
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