A strong earthquake has struck central Mexico, killing more than 140 people and toppling dozens of buildings in the capital, Mexico City.
President Enrique Peņa Nieto said more than 20 children died and 30 are missing after a school collapsed.
The 7.1 magnitude quake also caused major damage in neighbouring states.
The tremor struck while many people took part in an earthquake drill exactly 32 years after a quake killed thousands in Mexico City.
The country is prone to earthquakes and earlier this month an 8.1 magnitude tremor in the south left at least 90 dead.
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 said.
At least 149 people have died across the country, a civil protection agency spokesman said.
At least 55 people were killed in Morelos state, south of the capital, and 32 reported killed in Puebla state. Forty-nine are confirmed dead in Mexico City, with another 10 in Mexico State, and three dead in Guerrero.
About two million people in the capital were without electricity and phone lines were down. Officials also warned residents not to smoke on the streets as gas mains could have been ruptured.
Mexico City Mayor Miguel Angel Mancera told TV network Televisa that rescue services were dealing with collapsed or badly damaged buildings at 44 locations.
President Peņa Nieto said more than 20 children and two adults had been found dead at the collapsed Enrique Rebsamen school in Mexico City's southern Coapa district. He said another 30 children and eight adults were missing.
A six-storey blocks of flats, a supermarket and a factory were also said to be among the collapsed buildings in the city.
In a televised address, the president said an emergency had been declared for the affected areas and the military was being drafted in to help with the response.
"We may still find people under the rubble. Stay in touch, follow lines of communication, we will keep people updated," he said.
Across Mexico City, teams of rescue workers and volunteers clawed through the rubble with picks, shovels and their bare hands.
"My wife is there. I haven't been able to communicate with her," said Juan Jesus Garcia, 33, choking back tears next to a collapsed building.
"She is not answering and now they are telling us we have to turn off our mobile phones because there is a gas leak."
The earthquake drill was being held in Mexico City on the 32nd anniversary of a quake that killed up to 10,000 people.
Earthquake alarms did sound, correspondents say, but some residents apparently thought they were part of the day of drills.
Mexico City is one of the most densely populated cities in the world, with more than 20 million people living in the metropolitan area.
The prolonged tremor hit at 13:14 local time (18:14 irish Time) and sent thousands of residents into the streets.
Jennifer Swaddle, a teacher at the British International School in Mexico City, told the BBC that part of her classroom collapsed after the earthquake hit.
"Something that started as a tremor quickly escalated into something where the classroom shook," she said.
"As we were leaving, the outside of my classroom wall fell, so there was a big pile of rubble. Luckily, fantastically, nobody was hurt, but it was incredibly frightening."