Oslo’s governing mayor has called for a full investigation after dozens of teenage girls sustained minor injuries at a free Justin Bieber concert in the city centre on Wednesday evening, while the city's mayor hid behind a tree to avoid a stampede.

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The head of the city’s government, Stian Berger Røsland, said he was terrified of the risk for serious injuries as a crowd made up mostly of young girls thronged outside the opera house to watch the 18-year-old Canadian superstar perform.

At one point earlier in the day, Mayor Fabian Stang was forced to take shelter behind a tree to avoid being crushed by hordes of stampeding girls.

“I have already called on the Emergency Planning Agency to examine the entire event from the planning stage to its implementation. We have to find out what went wrong and why it happened.”

Fortunately, no one was seriously injured when thousands of frantic Justin Bieber fans arrived at the Opera House to watch Justin Bieber perform. Ola Krokan at the Oslo police told news agency NTB that 49 girls had received minor injuries while 14 were taken by ambulance to the emergency services, none of them with serious injuries.

Stian Røsland Berger said he had been fearful that children and young people would be crushed, pass out from dehydration, or tumble into the waters of the adjacent fjord.

“There’s no doubt that last night was a miscalculation – a dramatic miscalculation,” said the head of the city council.

Mayor Fabian Stang was also deeply critical of the organization of Bieber’s promotional visit. Earlier on Wednesday, Stang had been cycling down Stortingsgata when he found himself surrounded by hundreds of so-called Beliebers.

“Suddenly there was a huge bunch of girls running. I was scared and had to hide behind a tree to avoid being trampled. I have a great level understanding for dedication, but right there and then I was very scared,” Stang told VG.

“I was very worried about the young girls. It's not the girls' fault; the responsibility lies with the organizers.”  

Outside the Opera House, some 100 police officers and security guards fought hard to keep control of the masses of screaming teenagers.

“We were very worried,” said Kåre Stølen, the head of Wednesday night’s police operation. “There were a lot more people than we had expected. The ones who didn’t make it in to the area were both angry and disappointed.”

When Bieber had left the stage and thousands of fans streamed away from the area, Stølen reflected on what he viewed as a stressful but successful evening.

“It has gone very well. Now we just have to get everyone home safely,” he told NTB.

Universal's Norway chief Petter Singsaas said company representative had lumps in their throats before and during the event.

“We never dreamed that this would set off such a major movement of people. I think Bieber’s crew also felt a lot of pressure during the day even though they travel around with him daily and are more used to this kind of attention. There were extreme reactions in the city, and the logistics were complicated.”

Singsaas said he fully understood the concerns people had, not least the parents of Bieber fans, but he also felt Universal had brought joy to many fans on Wednesday evening.

He added that the company had worked closely with the police and security guards throughout, but said he fully expected the mayor and the council chief to call on Universal to explain why the situation had become so chaotic.

Picture special: Fan frenzy as Justin Bieber comes to Oslo