The quake struck beneath the Pacific Ocean off the west coast of the Central American neighbors at 9:03 p.m. local time at a depth of 50 miles (80 km), according to the U.S. Geological Survey (USGS).
The epicenter was placed 65 miles (110 km) south of Guatemala City and 95 miles west of San Salvador, the USGS Web site said.
Radio reports said the shaking caused widespread alarm, but residents soon returned to their homes for the night after no strong aftershocks occurred.
Guatemala disaster agency spokesman Benedicto Giron told the Associated Press that power and telephone service were briefly interrupted following the quake.
