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.

“We kill them so they don’t get close to the girls,” Elizabeth Noriegas said of the roaches as she waited with her daughter, Frida Coss, the 17-year-old president of the Official Justin Bieber Fan Club of Mexico City. Frida was first in line.

“Naturally we’d be worried about safety, but it looks like the government is taking the necessary precautions,” said Monica Gomez, mother of Renata, 11, who will miss school Monday as she waits in line.

Authorities said they will have more than 5,000 police on hand for the concert on the Zocalo, helping keep order around barricades and checkpoints meant to prevent a dangerous crush of fans. Most will have to watch Bieber on a series of large screens set up near the plaza, rather than inside with their idol.

A city official, Hector Antunano, said fans in the encampment would have a first shot at getting in as recognition for the effort they have made.

He said the city was determined to prevent problems such as the crush that injured 40 Bieber fans at a free concert in Oslo, Norway, in late May.

“Most of the fans will be between 10 and 17 years old. There will be a lot of girls. We are being very careful that the majority of the police are women and we are taking precautions so that there is no rush toward the stage,” Antunano said.

Leslie Bio, 13, her mother and three friends travelled 14 hours on a bus from the state of Sinaloa to attend the concert.

Justin “teaches us not to give up on our dreams,” Leslie said.

Only a block or two away, the city was, as often, in political turmoil.

An estimated 90,000 protesters filled the streets with other chants as they marched near the plaza known as the Zocalo, where Bieber is to perform. Many were opposing the possible return of Mexico’s old ruling party ahead of Sunday’s final, crucial presidential campaign debate, and many planned to watch the debate on giant open-air screens in and around the plaza. Some were members of a new student movement. Others commemorated a 1971 massacre of young protesters.

City officials initially promised to close off the entire Zocalo starting Saturday night, but apparently dropped the idea. That would have blocked access to the symbolic heart of the nation, with the National Palace, the Metropolitan Cathedral and the partially excavated remains of the Aztec capital, Tenochtitlan, all a few yards from where Bieber will perform.

Bieber fans stretched along a two-block stretch of sidewalk on 20 de Noviembre street as cars, trucks and buses growled by and the city’s ever-enterprising vendors passed among their tents selling posters and buttons of the singer.

A similar concert in the Zocalo last month by former Beatle Paul McCartney drew 230,000 people, Antunano said, including President Felipe Calderon. There was no word Sunday on whether the president would take his family to see the Bieber.