Hundreds living in isolated villages have been completely cut off by the earthquake, which also caused damage to buildings hundreds of miles away in Mexico City.
The fatalities were near the quake’s centre in Oaxaca, a mountainous state known for its coffee, mescal and Spanish colonial architecture.
A witness in the tourist town of La Crucecita, which Mexican authorities said was the epicenter of the earthquake, saw anxious residents standing outside their homes on the streets many hours after the tremor as they feared deadly aftershocks.
Many houses were scarred by huge cracks across walls and residents sought to clear debris from the streets, the witness added.
About 200 houses in the area were damaged, including 30 that were badly impacted, a local official said.
“We lost everything in one moment to nature,” said Vicente Romero, an owner of a stationary store whose house suffered vast structural damage. “This is our life’s work.”
Rockfalls blocked the winding mountain roads between the state capital of Oaxaca city and the coast.
Rescue workers have not reached all villages reported to have suffered damage, raising fears that more people may be hurt.
A Oaxaca state official said rescue workers were trying to get to the settlement of Santa Catarina Xanaguia, near the epicenter, where the quake brought down homes or parts of the mountainside, trapping people.
People had sent messages for help by phone, the official added.
A clinic and old churches in hill villages near the epicenter were severely damaged, images on social media showed.
The dead included a worker from state oil company Pemex, who fell from a height. Pemex was forced to briefly shutter the country’s biggest oil refinery in Oaxaca.
Miguel Candelaria, 30, was working at his computer in his family home in the Oaxaca town of Juchitan when the ground began to tremble.
He ran outside with relatives, but they had to stop in the middle of the street as the pavement buckled and rocked.
“We couldn’t walk… the street was like chewing gum,” said Candelaria, 30.
Quakes of magnitudes over 7 are major earthquakes capable of widespread, heavy damage.