The shaking toppled buildings and created a small tsunami, which did no damage.
Cable news station Canal N reported that the quake had caused a church to collapse in the city of Ica, south of Lima, killing 17 people and injuring 70 others.
Bodies of many more victims were discovered as rescue crews searched through the debris of other collapsed structures.
Buildings in the capital rocked violently at 6:40 p.m. local time, sending hundreds of people rushing into the streets.
The U.S. Geological Survey determined the earthquake was centered about 90 miles (145 km) southeast of Lima at a depth of about 25 miles (41 km).
The agency initially said the quake had a magnitude of 7.9, but the strength was upgraded to 8.0 late Thursday based on more detailed analysis of seismic data.
That strength makes it a "great earthquake," comparable to some of the most destructive seismic events in Earth's history.
Scores of strong aftershocks ranging from magnitudes of 5.4 to 5.9 were felt during the next several hours.
The last time a quake of magnitude 7.0 or higher struck Peru's central coast was in 1974 when one of magnitude 7.6 hit in October, followed by a 7.2 magnitude quake a month later.
Wednesday’s quake occurred in a subduction zone where one section of the Earth's crust dives under another, according to geophysicist Dale Grant at the U.S. National Earthquake Information Center in Golden, Colorado.
Some of the world's biggest quakes occur in subduction zones, including the catastrophic Indian Ocean temblor in 2004 that generated a deadly Indian Ocean tsunami.