"The boy is a good example," Noriegas said. "He's healthy, he's cute, he's a good singer and his music is not scandalous."
Justin "teaches us not to give up on our dreams," said Leslie Bio, who like nearly all in the line was wearing Bieber's favorite color, purple.
The 13-year-old, her mother and three friends travelled 14 hours on a bus from the state of Sinaloa to attend the concert.
Only a block or two away from where the headphone-wearing Bieber fans danced, chatted and chanted "Justin! Justin!", the city was, as often, in political turmoil.
Tens of thousands of 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.
The biggest challenge for the campers appeared to be the cockroaches that crawl about at night as they bed down in sleeping bags.
"We kill them so they don't get close to the girls," Noriegas said.
Authorities said they will have more than 5,000 police on hand for the concert, 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 that fans in the encampment will 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, last October.
"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," he said.
A similar concert in the Zocalo last month by Paul McCartney drew 230,000 people, Antunano said.