The epicenter was Chiapas state near the Mexico-Guatemala border with a depth of 35km.
Buildings swayed and electricity supply was disrupted. Effects were also felt in the faraway Mexico City.
Tsunami warnings have been issued for Mexico, EL Salvador, Guatemala, Costa Rica, Panama, Nicaragua and Honduras.
Powerful earthquake hits Mexico
90 million people residing in affected areas
By initial estimates, this is the strongest quake to hit the country since 1985, when a devastating tremor killed thousands.
People in Mexico City, Quetzaltenango and even Belize and Guatemala were seen running out of their homes as power went off.
The European-Mediterranean Seismological Centre says about 90mn people reside in the "felt area".
Hurricane Katia is meanwhile ravaging Mexico's eastern coast.