There was no immediate warning of any tsunami.
The quake occurred at 1:29 am (0729 GMT), epicentered 156 kilometers (97 miles) west of Guatemala City in the department of San Marcos, the Institute of Seismology said.
Separately, the US Geological Survey (USGS) measured the quake at 6.9 magnitude.
The tremor was felt across most of the country, including the capital, according to Guatemala's National Coordinator for Disaster Reduction.
"Moderate damage is reported to houses in southwestern areas and there are power outages in several parts of the country," agency spokesman Julio Sanchez said, adding that a 5.6-magnitude aftershock hit across the border in Mexico minutes after the first quake.
President Jimmy Morales posted a message on Twitter saying "stay calm and be alert to possible aftershocks."
Guatemala is in a risk zone for tremors, located where the Caribbean Cocos tectonic plates converge with North America's tectonic plates.
San Marcos was hit by earthquakes in 2012 and 2014 that left dozens dead.
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