Police said they had no record of any fatalities but divers went into the sea to make sure there were no bodies. Firemen also used thermal cameras to scan the wreckage for anyone trapped.
Those who were saved from the fall helped the injured right away.
Festival-goer Aitana Alonso told local paper Diario de Vigo: 'The floor gave way beneath us as if we were in a lift. It was a question of five seconds. The platform broke and we all fell.
"People landed on top of me. I tried to get out and couldn't. My foot ended up stuck in the water but I managed to free it before a boy gave me his hand and pulled me out.
"There were people underneath me, shouting they couldn't get out. I didn't think I would."
A friend described the scene as "horrible, chaos, panic", adding: "I saw a family and the little girl was saying to her dad, 'If we die here, I love you."
"It was the worst moment of my life," said one of the concert goers who reported seeing people screaming and running around.
Another 16-year-old said: "We were on the walkway when it opened up. I fell on people and people fell on me. I was hit on the head and then some sort of container fell as well. I tried to get out and someone helped me.
City mayor Abel Caballero has announced an investigation will now be launched to determine why the wooden platform collapsed.
Vigo's Port Authority president Enrique Lopez Veiga said he thought it was due to "excess weight".
There were no immediate report of any Britons or Irish among the casualties, although the nationalities of those hurt has not yet been revealed.
Members of the National Police, the Local Police and other emergency personnel immediately helped the injured with a succession of ambulances arriving on the scene. The injured were taken to various hospitals.
Organisers set up an information point for worried relatives who couldn't initially find their loved ones.
In a tweet, rap artist 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 organisers.
