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 225 people in total and caused major damage across states in the centre of the country.
Desperate searches for survivors under the rubble are continuing.
Dozens of buildings collapsed, including several churches where worshippers were killed.
Families with young children could be seen sleeping outside their homes in the street in the capital overnight.
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.
Enrique Rébsamen primary school collapsed in Mexico City’s southern Coapa district.
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.
“Some voices have been heard,” Mexican President Enrique Peña Nieto said after visiting the rescue operation.
“In the time I have been here, I have seen how at times they have asked for total silence, solidarity to listen for the voices.”
At least 209 schools were affected by the quake, 15 of which have suffered severe damage.
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:
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.
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.
Mexico is one of the most seismically active regions in the world, sitting on top of three of the Earth’s largest tectonic plates – the North American, Cocos and Pacific plates.
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.
Foreign leaders sent messages of support as the scale of the disaster became clear:
The Mexican government issued the following guidance on what to do if a quake strikes in the country:
The UK Foreign Office has said that those travelling to Mexico should follow the advice of the local authorities.
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