Thousands of Justin Bieber fans jamming a vast square screamed in unison when the teen superstar appeared on stage for a free concert Monday night, an event that was expected to draw 200,000 people to the historic centre of Mexico's capital.

Bieber opened with his hit song Baby, while adoring "Beliebers," who filled the city's main plaza, sang along or shouted "Justin! Justin! Justin!"

"Mexico City, there is a lot of people today!" Bieber told the crowd. "Every one of you is my biggest fan"

Bieber wore white jeans and a grey sweatshirt, which he took off to reveal another white shirt, and neon yellow tennis shoes. He was backed by five dancers in neon colours.

Four giant screens showed parts of his music videos to the crowd, which was comprised mainly of girls and their parents, who braved a light, intermittent rain during the concert.

Justin Bieber fans clutch pictures of the young pop star Justin Bieber on Sunday, a day ahead of his free concert in Mexico City's vast, historic Zocalo plaza. (Marco Ugarte/Associated Press)

The crowd went wild when Bieber asked a girl onto the stage, gave her a bouquet of flowers and sang One Less Lonely Girl into her ear. The girl didn't stop crying.

The plaza was a sea of purple as the mainly 10- to 14-year-old fans paid homage to what is reported to be Bieber's favorite colour.

At a news conference before the concert, the teen star said there is no artist he would spend days in line waiting for.

"There is no one I admire so much to do something crazy, but if Michael Jackson were here, I would do it for him. So, I do understand the emotion that the girls feel and that makes me feel very honoured," Bieber said.

Hundreds at the Zocalo, which was filled to capacity, had to cover themselves in plastic rain capes sold by vendors who came prepared for a rainy evening.

Earlier, girls shuffled through security checkpoints with hats and umbrellas, staking claim to some of the roughly 80,000 spots allocated in the plaza itself. An additional 120,000 or more fans were expected to watch on giant TV screens erected on nearby streets.

Fans yearned to get up close

City officials said more than 5,000 police — largely female officers — were on hand to maintain order among the young fans, to guard barricades and to generally prevent a dangerous crush like the one that occurred at a surprise Bieber concert in Oslo in May.A girl sleeps beneath a photo of Justin Bieber near Mexico City's central Zocalo plaza on Sunday. Diehard fans braved nights on roach-infested sidewalks for the chance to be closest to the stage for the teen star's free concert Monday evening. (Marco Ugarte/Associated Press)

Early arrivals camped out in sleeping bags on a stretch of sidewalk, forced to contend with nocturnal insects and cars, trucks and buses passing nearby.

Karina Gutierrez, a 13-year-old who uses a wheelchair, was disappointed with the spot authorities assigned to youngsters with disabilities.

She was the first fan to arrive in the special area that was set aside in one of the corners of the plaza, far from the stage.

"Visibility is not very clear from here," Gutierrez said.

She missed school and her parents didn't go to work so the whole family could attend the concert. Gutierrez arrived to the capital Monday morning with her parents and two brothers from the city of Toluca, about 70 kilometres from the capital.

"They wanted to see him from up close," said Gutierrez's father, Juan Manuel Salinas. "It's not fair that they are so far back."

Fernanda Gutierrez Aparicio, a 13-year-old seventh grader, said she spent a week camping out with her mother on a nearby street in hopes of being among the first to enter. She said they returned home only to bathe and look after Fernanda's 15 year-old sister, who recently had surgery.

'I told my teacher that I wasn't going to school and not to expect me in class for a few days, because I was going to be out supporting my idol.'—Fernanda Gutierrez Aparicio, 13-year-old fan

But her mother, Adriana Martinez, 41, gave up on trying to get her daughter into the front rows because of the press of other fans.

"I was really disappointed. When we got up front, people were crushing you, not letting you breathe," Martinez said. Even hours before the concert, "people were jostling each other and it got to the point that you couldn't move."

To Fernanda, it was all worth it — even missing the week of classes leading up to the two-hour concert that was to open with 3BallMTY and Canadian singer Carly Rae Jepsen.

"I told my teacher that I wasn't going to school and not to expect me in class for a few days, because I was going to be out supporting my idol," Fernanda said.

With files from The Associated Press