Rescuers and volunteers frantically dug through the rubble, looking for survivors of Mexico's deadliest earthquake since 1985 as the number of confirmed fatalities climbed to 248 early on Wednesday.
Tuesday's magnitude 7.1 quake struck on the 32nd anniversary of the tremor that killed thousands and came just two hours after earthquake drills were held acrossMexico to mark the date.
One of the most desperate rescue efforts was at a primary and secondary school in southern Mexico City, where a wing of the three-story building collapsed into a massive pancake of concrete floor slabs.
At the scene, journalists saw rescuers pull at least two small, sheet-covered bodies from the rubble.
The federal Education Department reported late on Tuesday night that 25 bodies had been recovered from the school's wreckage, all but four of them children.
It was not clear whether the deaths were included in the overall death toll of 248 reported by the federal civil defence agency.
During a visit to the site earlier in the night, President Enrique Pena Nieto had reported 22 bodies found and said 30 children and eight adults were reported missing at that point.
A mix of neighbourhood volunteers, police and firefighters used trained dogs and their bare hands to search through the school's rubble.
Reports swept through the crowd of anxious parents outside the gates that relatives in two families had received WhatsApp messages from girls trapped inside, but that could not be confirmed.
The rescue effort continued long through the night, the work punctuated by cries of "quiet" so searchers could listen for any faint calls for help.
"They have heard voices in there," Mr Pena Nieto said.
Rescuers had to shore up the fallen concrete slabs with wooden beams so they would not collapse further and crush whatever tiny air spaces remained.
In a video message released late Tuesday, Mr Pena Nieto urged people to be calm and said authorities were moving to provide help as 40% of Mexico City and 60% of nearby Morelos state were without power.
But, he said: "The priority at this moment is to keep rescuing people who are still trapped and to give medical attention to the injured people."
People across central Mexico already had rallied to help their neighbours as dozens of buildings tumbled into mounds of rubble.
Mexico City mayor Miguel Angel Mancera said buildings fell at 44 sites in the capital alone as high-rises across the city swayed and twisted and hundreds of thousands of panicked people ran into the streets blocking traffic.
Dust-covered and exhausted from digging, Carlos Mendoza, 30, said two people were pulled alive from the ruins of a collapsed building in the Roma Sur neighbourhood during a three-hour period.
"When we saw this, we came to help. This is ugly, very ugly," he said, gesturing at the destruction.
Blocks away, Alma Gonzalez was in her fourth-floor apartment when the quake collapsed the ground floor of her building, leaving her no way out.
She said she was terrified until the people living in the neighbouring house mounted a ladder on their roof and helped her slide out a side window.
Mr Mancera said 50 to 60 people were rescued alive by citizens and emergency workers in the capital.
The national Civil Defence agency reported early Wednesday that the confirmed death toll had climbed to 248, more than half of them in the capital.
The official Twitter feed of agency head Luis Felipe Puente said 117 dead had been counted in Mexico City and 72 in Morelos state, which is just south of the capital.
It said 43 were known dead in Puebla state, where the quake was centred.
Twelve deaths were listed in the State of Mexico, which surrounds Mexico City on three sides, and three in Guerrero state.
At the site of a collapsed apartment building in Mexico City, rescuers worked atop a three-story pile of rubble, forming a human chain that passed pieces of rubble across four city blocks to a site where they were dumped.
Throughout the day, rescuers pulled dust-covered people, some barely conscious, some seriously injured, from about three dozen collapsed buildings.
At one site, shopping carts commandeered from a nearby supermarket were used to carry water to the rescue site and take rubble away.
As night began to fall huge flood lights lit up the recovery sites, but workers and volunteers begged for headlamps.
Where a six-story office building collapsed in Mexico City, sisters Cristina and Victoria Lopez Torres formed part of a human chain passing bottled water.
"I think it's human nature that drives everyone to come and help others," Cristina Lopez said.
"We are young, we didn't live in '85, but we know that it's important to come out to the street to help," said her sister Victoria.
Ricardo Ibarra, 48, did live through the 1985 quake and said there had not been anything like it until now.
Wearing a bright orange vest and carrying a backpack with a sleeping bag strapped to it, he said he and friends just wanted to help.
"People are very sensitive because today was the 32nd anniversary of a tragedy," he said.
Buildings also collapsed in Morelos state, including the town hall and local church in Jojutla near the quake's epicentre.
A dozen people died in Jojutla.
The town's Instituto Morelos secondary school partly collapsed, but school director Adelina Anzures said the earthquake drill held in the morning came in handy.
"I told them that it was not a game, that we should be prepared," Ms Anzures said of the drill.
When the quake came, children and teachers rapidly filed out and nobody was hurt, she said.
The US Geological Survey said the magnitude 7.1 quake hit at 1:14pm and was centred near the Puebla state town of Raboso, about 76 miles southeast of Mexico City.
Much of Mexico City is built on former lake bed, and the soil can amplify the effects of earthquakes centred hundreds of miles away.
The quake appeared to be unrelated to the magnitude 8.1 tremor that hit on September 7 off Mexico's southern coast and also was felt strongly in the capital.
US Geological Survey seismologist Paul Earle noted the epicentres of the two quakes were 400 miles apart and said most aftershocks are within 60 miles