At least 48 people have been killed and some 20 are missing after a 7.4-magnitude quake hit off Guatemala's Pacific coast.
President Otto Perez Molina, who toured the worst affected areas, has declared three days of mourning.
Officials said landslides had buried roads and it would take 24 hours to restore links to the quake-hit area.
Frightened people fled from offices and homes around the region, as buildings shook from Mexico City to San Salvador.
The energy ministry said about 73,000 people were left without electricity.
Quake officials said the tremor hit on Wednesday, at about 10:35 local time (16:35 GMT) about 23km (15 miles) from the Guatemalan town of Champerico.
The director of Guatemala's Seismological Institute, Eddy Sanchez, said it had been the biggest tremor in Guatemala since 1976, when 25,000 people died in a 7.5-magnitude quake.
There have been dozens of aftershocks, four of them over a magnitude of 5.
President Perez Molina has given several news conferences, each time raising the number of casualties.
Touring the affected area, he said 40 deaths had been confirmed in San Marcos province and another eight in neighbouring Quetzaltenango.
He said 23 people were still missing and 153 were being housed in emergency shelters.
Worst-hit was the town of San Marcos where many buildings were damaged and more than 30 destroyed completely.
Speaking in San Marcos, Mr Perez Molina said: "It's very sad to meet people here who are waiting to find their families who are still buried.
"It's really a tragedy and we will do all we can to help the families that are suffering."
Residents spent Wednesday night outside, wrapped in blankets, while others took shelter in the hospital, the only building with electricity, the Associated Press reports from San Marcos.
The national volunteer fire service gave details on its Twitter feed of the damage across the San Marcos region, describing how firefighters had pulled seven bodies from the rubble, including that of an 11-year-old boy.
In the town of San Cristobal Cucho, 10 members of one family were killed when their house collapsed.
Fourteen houses have been destroyed in the city of Quetzaltenango, which houses dozens of language schools popular with tourists.
Panama, El Salvador and Honduras have offered their help with the rescue operation.