A "tween" squatter encampment has sprung up in the chaotic Mexico City center as Beliebers seek to be closest to the stage when teenage superstar Justin Bieber puts on a free concert Monday evening on the capital's vast central plaza.

Dozens of mostly girls aged 10 to 14 travelled for hundreds of miles to camp Sunday along a street near the Zocalo, where tens of thousands of unrelated political demonstrators gathered and crews erected the concert stage.

Many of the young campers, dressed in Bieber's favorite color of purple and bopping to his music on headphones, were chaperoned by parents, some of whom took time off work to help their children miss school and be part of an audience expected to top 200,000.

The line of hundreds, which broke out spontaneously in chants of "Justin," seemed more worried about the giant cockroaches that roam Mexico City's streets at night than security, even as a similar free concert by Bieber in Norway two weeks ago caused a stampede that injured dozens of fans.