A wooden boardwalk collapsed during a concert in the port city of Vigo, Spain, late Sunday, injuring at least 336 people who were attending a festival. Rescue workers say there was chaos when boards buckled and concert-goers began sliding into the sea.

"Witnesses reported scenes of panic as hundreds of people fell into the ocean around midnight, some falling right on top of each other," Lucia Benavides reports from Barcelona for NPR's Newscast unit.

Five people were seriously injured, including two who were still in intensive care on Monday morning, according to the regional Galician government website.

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.

The collapse occurred shortly after the concert headliner, Rels B, tried to pump up the crowd by telling people to jump, witnesses told local news site Faro de Vigo.

Many of those attending the show were 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.

Officials in Vigo, which sits in an estuary along the Atlantic Ocean, did not yet know what caused the boardwalk to collapse.

"The president of the city's port authority said he suspects structural problems were behind the collapse," Benavides reports. "An investigation into the case will determine who is responsible for the boardwalk's maintenance."

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