The president said the government would pay for the funerals of all victims in the impoverished region.
Efrain Ramos helped load a tiny casket carrying the body of his 6-year-old niece from San Marcos' morgue to a waiting pickup truck.
"She is my niece. The little girl died when a wall fell over her," a shocked Ramos told a reporter. He said the girl was playing in her room when the quake hit.
The girl's mother hugged the coffin wrapped with white lace and tulle while sobbing uncontrollably.
Ramos said the family would escort his niece Rosa's body back home for a viewing.
In the town of San Cristobal Cochu, firefighters were trying to dig out 10 members of one family, including a 4-year-old child, who were buried when their house collapsed, fire department spokesman Ovidio Perez told the radio station Emisoras Unidas.
Many of the colorful adobe buildings in the 10-square-mile center of San Marcos were either cracked or reduced to rubble, including the police station and the courthouse. The temblor left a large gash in one of the streets, and hundreds of frightened villagers stood in the open, refusing to go back inside.
Hundreds of people crammed into the hallways of the small town hospital waiting for medical staff to help injured family members, some complaining they were not getting care quickly enough.
Ingrid Lopez, who went to the hospital with a 72-year-old aunt whose legs were crushed by a falling wall, said she had waited hours for an X-ray.
"We ask the president to improve conditions at the hospital," she said. "There isn't enough staff."
The quake, which was 20 miles deep, was centered 15 miles off the coastal town of Champerico and 100 miles southwest of Guatemala City. It was the strongest earthquake to hit Guatemala since a 1976 temblor that killed 23,000.