At least 21 children and five adults died at a primary school which collapsed in Mexico City during Tuesday's quake, the government says.
The shallow, 7.1 magnitude quake killed at least 230 people in total and caused major damage across states in the centre of the country.
Desperate searches for survivors under the rubble are continuing.
Hopes rose at the school when a girl was located alive and a delicate operation to rescue her is under way.
Dozens of buildings collapsed across the country, including several churches where worshippers were killed.
Families with young children could be seen sleeping outside their homes in the street in the capital.
President Enrique Peña Nieto has declared three days of mourning.
The tremor struck shortly after many people had taken part in an earthquake drill, exactly 32 years after another quake killed thousands in the capital.
Mexico is prone to earthquakes and earlier this month an 8.1 magnitude tremor in the south left at least 90 people dead.
Though it struck a similar region, Tuesday's earthquake does not appear to be connected with the quake on 7 September, which was at least 30 times more energetic, the BBC's Jonathan Amos writes.
What do we know about the school disaster?
Enrique Rébsamen primary school, in Mexico City's southern Coapa district, collapsed.
Confirming the death toll, Education Minister Aurelio Nuño said 11 people had been rescued while two children and an adult were missing.
Local media had reported a higher death toll earlier.
More than 500 members of the army and navy, along with 200 police officers and volunteers, have been working at the site, Mexican newspaper Milenio says.
Details of the girl located on Wednesday have not been given. Rescuers detected her after she moved her hand and a hose was lowered to supply her with water.
Civil Protection volunteer Enrique Gardia told the assembled crowd that a thermal scanner had detected several survivors trapped between slabs of concrete.
"They are alive! Alive!" he shouted.
"Someone hit a wall several times in one place, and in another there was a response to light signals with a lamp," he added.
One mother, standing nearby waiting for news of her seven-year-old daughter, told reporters: "No-one can possibly imagine the pain I'm in right now."
At least 209 schools were affected by the quake, 15 of which have suffered severe damage.
Set on saving lives
By Rajini Vaidyanathan, BBC News, Mexico City
In the heat and humidity of Mexico City, there's also hope. Police officers guard a six-storey office and residential building in the Bohemian La Condesa neighbourhood. The concrete structure has been flattened to a towering mass of bricks and twisted metal.
The damage is so severe I had to look at the building next door, which remains intact, to get an idea of its neighbour's pre-quake state. Moments later a team of rescuers in blue overalls and red hats strap head torches on as they prepare to enter the building.
More than 50 people have been rescued in this city so far. People here are determined to make sure that it's that figure that climbs, and not the death toll.
As is often the case in the wake of a devastating natural disaster, people are also displaying resilience.
Hundreds of people are wandering the streets holding handwritten signs which offer water and food to people.
A man also holds a sign which says "no smoking". The earthquake has led to many gas leaks and that, as well as the fear of further aftershocks, remains a concern here.
Where else was hit?
The epicentre of the latest quake was near Atencingo in Puebla state, about 120km (75 miles) from Mexico City, with a depth of 51km, the US Geological Survey says.
The prolonged tremor hit at 13:14 local time (18:14 GMT) on Tuesday and sent thousands of residents into the streets.
Most of the victims died in the capital, according to government figures:
- Mexico City: 100 dead
- Morelos state: 69 dead
- Puebla state: 43 dead
- Mexico state: 13 dead
- Guerrero: Four dead
- Oaxaca: One dead
Fifteen people were killed when a church near Mexico's Popocatepetl volcano collapsed during Mass. The volcano itself had a small eruption as a result of the tremor.
In Atzala, Puebla, another church collapsed during a baptism, killing 11 people including the baby, Church officials told Efe news agency.
Alfredo del Mazo Maza, governor of the State of Mexico, said schools would be closed on Wednesday. He also ordered all public transport to operate services for free so that people could travel home.
What happened in 1985?
An earthquake drill was being held in Mexico City on Tuesday to mark the 32nd anniversary of a magnitude 8 quake that killed up to 10,000 people and left 30,000 others injured.
The severe tremor caused serious damage to Mexico City and its surrounding areas, with more than 400 buildings collapsed and thousands more damaged.
Correspondents say that residents may have mistaken earthquake alarms for part of the day of drills in the wake of the 1985 quake.
Mexico City is one of the most densely populated cities in the world, with more than 20 million people living in the metropolitan area.
Why is Mexico so prone to earthquakes?
Mexico is one of the most seismically active regions in the world, sitting where three of the Earth's tectonic plates - the North American, Cocos and Pacific plates - meet.
The latest tremor occurred near the boundary between the North American and Cocos plates, where the latter slides beneath the former.
According to the US Geological Survey, the country has seen 19 earthquakes of at least 6.5 magnitude within 155 miles of the epicentre of Tuesday's quake over the past century.
How has the world reacted?
Foreign leaders sent messages of support as the scale of the disaster became clear:
- US President Donald Trump, who has courted controversy with his plans for a border wall with Mexico, tweeted: "God bless the people of Mexico City. We are with you and will be there for you"
- Canadian Prime Minister Justin Trudeau also tweeted his support following the "devastating news"
- Costa Rican President Luis Guillermo Solis, in New York for the UN General Assembly, expressed his "solidarity" with the Mexican people
- Pope Francis said his thoughts and prayers were with the families of those who had lost loved ones in the "devastating" quake. "In this moment of pain I want to express my closeness and my prayer to all of the beloved Mexican population," the pontiff said at the Vatican
The Mexican government issued the following guidance on what to do if a quake strikes in the country:
- stay calm
- find the evacuation route and meeting point
- stay away from windows and objects that may fall
- in case of emergency call 911
- do not use lifts
The UK Foreign Office has said that those travelling to Mexico should follow the advice of the local authorities.