A strong earthquake has struck central Mexico, killing more than 139 people and toppling dozens of buildings in the capital, Mexico City.
Rescuers are searching for survivors and there are reports of children trapped in a partly collapsed school.
The 7.1 magnitude quake caused damage in Morelos and Puebla states and in Mexico State.
It struck while many people took part in an earthquake drill exactly 32 years after a quake killed thousands.
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 next to Atencingo in Puebla state, about 120km (75 miles) from Mexico City, with a depth of 51km, the US Geological Survey said.
At least 139 people had died across the country, a civil protection agency spokesman told the BBC.
At least 64 people were killed in Morelos state alone, south of the capital, and 29 reported killed in Puebla state. Thirty-six are confirmed dead in Mexico City, with another nine in Mexico State, and one 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.
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 GMT) 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."
A six-storey blocks of flats, a supermarket and a factory were said to be among the collapsed buildings in Mexico City.
Mexican media also reported that some children had been rescued from the partially collapsed Enrique Rebsamen school, in Mexico City's southern Coapa district, but that others were still trapped.
Across the 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."
President Enrique Peña Nieto urged people to avoid the streets so emergency services could reach the most affected areas.