Bieber opened with his hit song "Baby," while his adoring fans filling 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."
Dressed in white jeans and a gray sweatshirt that he took off to reveal a white T-shirt, the singer wore neon yellow tennis shoes and was accompanied by five dancers in neon colors. Four giant screens showed parts of his music videos to the crowd of mainly girls and their parents, who braved a light, intermittent rain during the concert.
The crowd went wild when Bieber asked a fan 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 gray and reddish stones of the plaza were covered in a sea of purple as the mainly 10- to 14-year-old crowd paid homage to what is reported to be Bieber's favorite color.
After singing a couple of songs while playing an acoustic guitar, Bieber changed into an all- black outfit and sang some of his more energetic songs, including "Somebody To Love."
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 honored," 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.
Mariana Villanueva, 12, traveled from the western city of Guadalajara for her idol's concert. She was near the stage, where she said she almost fainted because people kept squeezing her.
"It was worth it!" Villanueva said. "I have no words; it was magnificent!"
The concert closed with fireworks and confetti spewing from the stage.
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.
Fernanda Gutierrez Aparicio, a 13-year-old seventh grader at a local middle school, 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.
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.
Fernanda most wanted to hear Bieber sing "That Should Be Me," which pretty much reflects her feelings about the Canadian singer.
"I want to be Selena Gomez, to be with him," she cooed, referring to Bieber's girlfriend.
Authorities said they would have more than 5,000 police on hand, partly to prevent the sort of 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," said Hector Antunano, a city official. "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."
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 45 miles (70 kilometers) 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."
The Zocalo is ringed by some of the most historic structures in the hemisphere: Mexico's Metropolitan Cathedral, the National Palace and the partially excavated remains of the Aztec capital, Tenochtitlan.
The plaza was created by the Aztecs in 1325 and was redrawn after the Spanish conquest of 1521. Bieber's performance emptied the sprawling square of its normal population, heavy on tourists, street vendors and political protesters, some of whom agreed grudgingly to clear space for the concert by abandoning an encampment there after negotiations with the city government.
A similar concert in the Zocalo last month by former Beatle Paul McCartney drew an estimated 230,000 people, including President Felipe Calderon. There was no word on whether the president would take his children to see Bieber.