Authorities have confirmed that 313 people were injured in the collapse.
There were no reports of any fatalities from the incident which broadcaster Radio Galega said left 266 people injured at the O Marisquino festival in Vigo, a town in the Galicia region.
Five people were seriously injured, although their injuries were not life-threatening, and a search and rescue effort continued throughout the night.
Vigo mayor Abel Caballero announced an investigation.
Many of those attending the show are teenagers; as part of the emergency response, organizers and aid workers helped reunite them with their parents and share details about which patients had been taken to which hospitals, Faro de Vigo reports.
Local daily El Faro de Vigo said the structure collapsed shortly after Spanish rap artist Rels B told the crowd to jump at the start of his concert. "A boy gave me his hand in the end and I got out".
"I had people underneath, screaming that I could not get out, I was overwhelmed, I thought I was not going out, I was trying and falling down again".
Wooden boards cracked along a central section of the walkway before people tumbled into the water, emergency services have said.
"The vast majority are light injuries for bruises".
The annual festival, which bills itself as the "most important urban sports event in southern Europe", draws some 160,000 people to Vigo, a city of around 300,000 residents.