A strong earthquake has struck central Mexico, killing more than 130 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.
Panic on the streets
Mexico City is a city all too used to earthquakes. But this tremor, on the anniversary of another one that left thousands dead in 1985, was especially powerful.
It sent thousands of people into the streets, trembling, shaking, crying, and trying to reach their loved ones by phone.
As time passes it is becoming clear that there are going to be many victims. In the capital alone, about 30 buildings collapsed.
In one of the worst-affected areas I saw dozens of people desperately removing rubble because they believed someone was trapped.
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.
Foreign leaders sent messages of support to Mexico 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".