A seaside jetty has collapsed during a music festival in the north-western Spanish city of Vigo, injuring 313 people, five of them seriously.

Many young people were on the port’s wooden platform when it gave way around midnight on Sunday (Monday 8am AEST) at the closing event of a three-day O Marisquino festival, officials said.

The wooden boards cracked along a central section, about 40 metres long, and people slid down into the sea below, emergency services said. Eyewitnesses reported scenes of panic.

The head of the Galician region’s health authority, Jesus Vazquez Almuina, told Antena 3 television that 313 people required treatment.

Accidente en #OMarisquiño

Toda la información sobre el suceso que empaña la despedida del festival la tenéis aquí https://t.co/UPri79EdjM pic.twitter.com/t3w2neiDLg

— Faro de Vigo (@Farodevigo) August 12, 2018

The five people who were seriously hurt did not have life-threatening injuries, he said.

The incident happened shortly as the crowd was listening to the Majorcan rap artist Rels B.

Investigators believe structural issues on the platform were to blame for the accident. Photo: Miguel Fidalgo/Twitter

Fans of the festival were quick to express their sympathies on social media.

@omarisquino #vigo so sad to wake to the news. We were here at the festival yesterday early evening to watch the bmx ; it was brilliant. Super friendly atmosphere and we all were reassured by the police presence. The event was well organised and this is a terrible accident.

— Becky Kelway (@RKelway) August 13, 2018

My solidarity with those injured in the bridge collapse at this year's #OMarisquiño music fest in the port of Vigo. Years of neglect + big crowd = disaster.

— Adrian McManus🇪🇺 🇪🇸 (@MacMaghnusa1960) August 13, 2018

The president of the Vigo port authority, Enrique Cesar Lopez Veiga, said he suspects there was a structural problem in the platform, which is supported by concrete pillars.

He told Cadena Ser radio the platform collapsed “probably because of excessive weight” on it.

Vigo mayor Abel Caballero announced an investigation.

-AAP