Justin Bieber's Norwegian concert was so chaotic the mayor hid behind a tree "to avoid getting trampled".
According to website TMZ, 14 teenage girls had to be taken away for "emergency care" after hysteria over the pop star broke out at the Oslo show. It is understood 49 fans were injured.
While no one was seriously hurt, Oslo Mayor Fabian Stang has launched an investigation "to find out what went wrong and why it happened".
Stang says: "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.''
Police very nearly declared a state of emergency as hundreds of Bieber fans descended onto the town, screaming excitedly for their idol.
Video from Oslo's centre shows hordes of teenage girls running across roads and dodging cars to follow a vehicle they believed contained the Boyfriend singer.
"Mayor Stang said it got so bad ... he had to hide behind a tree at one point to avoid getting trampled," TMZ writes.
Bieber was in Norway to perform four new songs outside Oslo's famed opera house as part of his Around the World TV special, attracting crazed fans from all over Europe.
Some of his loyal followers reportedly even tried to get close to the star by paddling across the harbour on inflatable rafts.
The 18-year-old heartthrob appealed to his fans to stay safe via Twitter.
"Norway - please listen to the police. I dont want anyone getting hurt. I want everything to go to plan but your safety must come first ... for the show to happen u must all listen to the police," he wrote.
"We are all concerned for your safety and i want what is best for u. please listen and tonight we will have some fun. love you and im excited to see you all (sic)."
The concert went ahead as planned despite crowd control concerns - Bieber performed his hit Baby and four other songs.
But the concert appears to have taken its toll on the singer, who tweeted the followiong morning that he'd injured himself.
"Dear world... (I'm) being told I have to tweet to tell you what's up. I'm grumpy. Just woke up. Being honest... I pulled something in my hip yesterday."
Bieber is due to continue his tour to France.
Watch video footage here: