Rescuers are desperately searching for survivors amongst the rubble of collapsed buildings in Mexico City.
More than 200 people have been killed in the 7.1 magnitude quake - the most deadly to hit the country for 32 years - and which occurred on the anniversary of the 1985 quake which killed thousands.
One of the most desperate rescue efforts has been at the Rebsamen primary and secondary school, where a wing of the three-story building collapsed into a massive pancake of concrete slabs, killing dozens of students.
Mexico's civil defence agency has lowered the confirmed death toll to 223, down from 248.
Earlier Pedro Serrano, 29, a doctor, managed to crawl inside a classroom at the school, only to find its occupants dead.
"We saw some chairs and wooden tables," he said.
"The next thing we saw was a leg, and then we started to move rubble and we found a girl and two adults - woman and a man."
Similar scenes were repeated at dozens of collapsed homes, schools and offices.
The Mexico City government said 52 people had so far been rescued from the rubble of collapsed buildings.
The city's Social Development Department tweeted the number on Wednesday afternoon and added: "We won't stop".
At least 93 people have died in Mexico City, 69 in Morelos state, 43 in Puebla, 13 in the State of Mexico, four in Guerrero and one in Oaxaca, National Civil Defence Coordinator Luis Felipe Puente said.
Rescue workers repeatedly stopped and called for quiet in order to hear anyone who might be calling for help inside.
The federal interior minister, Miguel Angel Osorio Chong, said search efforts had to be carried out slowly and painstakingly slow because of the fragility of rubble.
"It has to be done very carefully," he said, adding: "time is against us".
President Pena Nieto issued a video statement urging people to stay calm.
He said many people will need help, but the initial focus has to be on finding people trapped in wrecked buildings.
The tremor came hours after many residents carried out disaster drills on the anniversary of the 1985 earthquake.
Mexico-based journalist Natasha Pizzey said that may have added to initial confusion as some people thought they were more tests.
"I think a lot of people didn’t realise it was the real thing, they thought it was part of the drill when alarms started going off," she said.
"It is devastating. It’s a very scary and sad day for Mexico."
Pope Francis led prayers for Mexico, as he led tens of thousands sending their well-wishes to those affected.
"In this moment of pain, I want to express my closeness and prayer to the dear Mexican people," he told worshippers at his weekly audience in Rome.
He urged prayers for victims, their families and rescue crews.
Theresa May has also said her "thoughts and sympathies" were with all those affected by the earthquake.
"The UK Government is in close contact with the relevant authorities and we are ready to provide support in any way that we can," she said in a statement.
"We are working to identify and support any affected British nationals as the recovery effort continues."
The UK Foreign Office said Britons in need of emergency assistance should contact +44 (0)20 7008 1500.
It advised anyone due to fly out of the county today to contact their their airline to see if the flight is still going ahead as planned.
The new earthquake appeared to be unrelated to the magnitude 8.1 quake that hit on 7 September off Mexico's southern coast and also was felt strongly in the capital.
USGS seismologist Paul Earle noted the epicentres of the two quakes were 400 miles apart and said most aftershocks are within 60 miles.
Much of Mexico City is built on former lakebed, and the soil is known to amplify the effects of earthquakes even hundreds of miles away.
Electricity and mobile phone service was interrupted in many areas and traffic was snarled as signal lights went dark.
Pena Nietosaid that as of late Tuesday 40 per cent of Mexico City and 60 per cent of Morelos state have no electricity.
The US Geological Survey (USGS) said the magnitude 7.1 quake hit at 1:14pm (7.14pm BST) with an epicentre near the Puebla state town of Raboso, about 76 miles southeast of Mexico City.