The number of confirmed dead in Mexico City rose to 100 as the nation's capital bore the brunt of the deaths and damage in Tuesday's quake.
Emergency workers battled through the night to try to free a young girl trapped in the rubble of a collapsed school.
She was found in the debris at the Enrique Rebsamen school in a southern area of the capital - where at least 21 children died and a further 30 remain missing.
Local TV reported that rescuers spotted the child on Wednesday and a search dog subsequently entered the wreckage and confirmed she was alive.
Her fingers were later seen wiggling as teams tried to get to her.
Emergency services have been joined by Mexico's "mole" rescue workers, who formed as a volunteer search and rescue group in the aftermath of the devastating quake that struck Mexico City in 1985.
They have worked at disaster sites across the world including the 2001 attacks on the World Trade Center in New York.
Oscar Guevara, a doctor from the central Mexican state of Queretero who joined the Topos of Tlatelolco search and rescue group in 2010, said they had found three people alive, along with some bodies, in the rubble of one building.
President Enrique Pena Nieto has declared three days of national mourning, but said the race to pull out survivors was still the priority.
"Every minute counts to save lives," he tweeted.
The quake was centred near Raboso in Puebla state, 76 miles (123km) southeast of Mexico City, but the shaking in the capital was almost as intense.
When it struck, panicked workers fled from office buildings and clouds of dust rose up from the crumbling facades of damaged buildings.
The earthquake came less than two weeks after an 8.1 magnitude tremor in southern Mexico killed at least 98 people.
Amid the latest quake, the Popocatepetl volcano near the capital had a small eruption, flattening a church on its southern slopes and killing 15 people attending a mass.
