At a collapsed school, where the bodies of nearly two dozen school children were found, teams made contact with a trapped girl. She was alive.
"The priority now is continue rescuing those who are still trapped and provide medical attention to the injured," President Enrique Peña Nieto said, calling
Tuesday's quake -- the second to shake Mexico in 12 days
-- "a new national emergency."
The magnitude-7.1 quake turned dozens of buildings in central Mexico into dust and debris, killing at least 250 people, Mexico City Mayor Miguel Angel Mancera told CNN affiliate Foro TV.
It occurred at a depth of 51 kilometers (32 miles), which experts consider to be shallow. Shallow quakes tend to be more destructive.
"Unfortunately, many people have lost their lives, including girls and boys in schools, buildings and houses. I want to express my condolences to those who lost a family member or a loved one. Mexico shares your grief," Peña Nieto said.
According to Luis Felipe Puente, national coordinator of civil protection for the Interior Ministry, 100 of the deaths came in Mexico City, which, with an urban area of more than 21 million people, is one of the most populous cities in the Western Hemisphere.
There were also 69 deaths in Morelos state, 43 in Puebla state, 13 in the state of Mexico, four in Guerrero state and one in Oaxaca state, according to Puente's tally. To provide some scope of the affected area, Oaxaca de Juarez, the capital of Oaxaca state, is almost 480 kilometers (300 miles) from Mexico City.
The country has declared three days of mourning, according to Mexico's secretary of public function.
Schools hit hard
Twenty-five bodies, 21 of them children
, were discovered in the debris of the Enrique Rebsamen school in Mexico City's Coapa district, Puente said in a tweet.
At least two children and one adult were still missing Wednesday, Secretary of Public Education Aurelio Nuño said on Twitter. Eleven people have been rescued from the school, he said.
The nation watched late Wednesday as rescuers made contact with the girl trapped in the rubble of the collapsed school. The rescue teams gave her oxygen and water, Foro TV reported. The girl "feels" two other students were close to her, but she didn't know whether they were alive, Foro TV reported.
Those gathered around were silent, and several priests dressed in white stood off to the side.
Photos of missing students circulated on social media, while dozens of parents waited outside the school hoping to find them. The family of 7-year-old Jose Eduardo Huerta Rodriguez looked for hours through handwritten lists with the names of those who had been rescued. They also visited the city's hospitals.
Late Tuesday, a family member who had stayed outside the school called Jose's mother.
"He was still inside the school, and he was dead when they rescued him," his aunt Paola Rodriguez told CNN.
Search and rescue teams pulled one person from a collapsed building in the Lindavista neighborhood of Mexico City, Interior Secretary Miguel A. Osorio Chong tweeted.
More than 2,000 public schools suffered damage in the quake, the education secretary said. Sixteen of the 212 affected schools in Mexico City had serious damage, he said.
Four people were killed and 40 more injured at the Monterrey Institute of Technology campus in Mexico City, the school said.
An unaccounted number of people are staying at shelters around the city after losing their homes. Schools have closed indefinitely, and millions remain without power.
'People screaming, children everywhere'
Peña Nieto asked people to stay indoors and stay away from the streets while the rescue attempts continue. Still, residents are joining forces with rescue workers to search for survivors.
Mexico's soccer teams are helping in the relief effort, with C.F. Monterrey, Tiburones Rojos de Veracruz, C.F. Pachuca, Santos Laguna, Puebla F.C. and Club Atlas urging people to bring water and nonperishables to stadiums and other facilities.
In the upscale district of Roma, one of the hardest-hit areas of Mexico City, Violeta Gaytan was among those who fled to get away from damaged buildings and a strong smell of gas.
"There was a lot of people screaming, children everywhere. I saw people calling out for doctors and a lot of dust," the publicist said.
Like others, Gaytan began to walk home. She expected it would take hours, but a man was driving around offering people rides to metro stations across the city.
Hours before Tuesday's earthquake, authorities held a citywide drill on the anniversary of the 1985 earthquake that killed an estimated 9,500 people
in and around Mexico City, one the country's worst natural disasters.
Less than two weeks ago, a magnitude-8.1 earthquake struck off Mexico's southern coast, killing at least 90 people, according to the governor of Oaxaca state.