Several people were injured at a free JLS concert in Birmingham, England's city square, after fans trampled the fences and rushed into the already-at-capacity outdoor venue.

According to local reports, authorities expected 5,000 fans to attend the event, during which JLS was to lend a hand in turning on Birmingham's Christmas lights. However, an estimated 20,000 people showed up and were admitted into the fenced-off square, according to the Sunday Mercury news.

Despite the organizer's ability to accommodate the unexpected fans, there remained a large crowd of people gathered outside of the fenced concert area who became frustrated when they were denied entry, according to a Sky News report. The spectators apparently pushed on the fence until it gave way and began running toward the stage, creating a crowd surge in which fans essentially were trapped.

"The frenzy started and people started running towards the front and we got caught within the crowds, so we had no choice but to go with the flow," one woman at the concert told Sky News. "It was extraordinarily terrifying for myself and my daughter."

One woman suffered serious injuries to her pelvis, shoulder and leg, according to the Sky News report. She was taken to a local hospital after a medical team immobilized her for a safe transport with a neck collar, spinal board and leg splint.

A teenage girl reportedly suffered injuries to her back and legs and at least two others sustained possible fractures.

More than 50 people with minor injuries were treated by ambulance crews at a temporary triage unit.

Officials pulled the plug on the show--which also was to feature The Sugarbabes--saying the stampede of young pop fans who turned up to see the boy band created a safety issue that could not be contained if the concert proceeded, according to Sky News.

A spokesperson for the Leisure Sport and Culture division of the Birmingham Council said he fully supported the decision to cancel the concert because of the public-safety issue. "I'm sorry a lot of people went home disappointed and there will be a full investigation into what went wrong ... but I must stress that this was the correct decision in the circumstances."

In addition to JLS and The Sugarbabes, the concert was to feature Tinchy Stryder, Chipmunk, Little Boots, Alexandra Burke, the Noisettes and Pixie Lott.