Three of the youngsters had fractures while the others were less seriously injured, said spokesperson Marita Broberg at Norrland University hospital.
The floor, based on a raised steel construction, gave way under the feet of a large group of people dancing in front of the stage in the Black Box room of the Norrland Opera House. Audience members fell over 2 metres to the floor below.
“The spectators landed on each other when they fell through the hole,” said emergency services spokesman Christer Björkman.
On Sunday morning the chief executive of the Norrland Opera House, Magnus Aspegren, said that the cause of the accident was that one or more of the metal beams in the floor construction had broken. But it was still unclear why.
“It’s a type of beam which should hold much more weight than this,” said Aspegren.
The floor consists of metal beams covered by removable wooden panels, to allow entrance to the stage through the floor from the basement during performances.
“Each of the wooden panels should be able to hold over a tonne, so it’s quite remarkable that it broke like this,” said Magnus Aspegren.
Before the accident the floor was bouncing under the feet of those who were dancing, according to one audience member.
The collapse happened just before 11.30pm on Saturday, barely 15 minutes into a concert by Swedish rock band Mando Diao.
“A total of 815 people were in the venue,” said police spokesman John Forsberg.
The worst injured had broken arms, lacerations and similar wounds. Around 20 people had less serious injuries and were suffering from shock.
According to local newspaper Västerbottens-Kuriren, the concert was part of the Amplified festival.
“The cafeteria was turned into cottage hospital,” said musician Markus Styrman, whose band Karavan was due to play at the festival after Mando Diao.
“A couple of the injured people were taken directly to hospital but many were lying on the cafeteria floor for a while, waiting for an ambulance.”
“It was obviously a complete surprise but there was hardly any panic even if everyone wanted to get out as quickly as possible,” said Styrman.
The floor consisted of large black panels, measuring 4.5 metres by 2.5 metres, sitting on a steel construction. The panels are around 5 centimetres thick.
One of the panels collapsed and fell 2.3 metres to the real floor below, said a police investigator. The panel had come loose from the supporting beams as the audience danced during the concert.
Two other panels had also wobbled loose and looked as though they could collapse at any time, according to TT’s reporter at the scene.