The quake, originally estimated to measure 6.6 on the Richter scale before being revised upward, occurred at 1:29 am (0729 GMT).
The epicenter of the quake was about 156 kilometers (97 miles) west of Guatemala City in the department of San Marcos, the Institute of Seismology said.
The tremor was felt across most of the country, including the capital, as well as in neighboring Mexico, according to Guatemala's National Coordinator for Disaster Reduction.
A man was killed in the southwestern Guatemalan village of San Sebastian after an old church collapsed, the agency said.
News reports said another victim, a woman, died of a heart attack in a small village near the border with Mexico.
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.
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