A prolonged 7.2 magnitude quake has hit Mexico leaving nearly a million homes and businesses without power in the capital and south of the country.
The only reported deaths so far came when a military helicopter crashed while surveying the aftermath.
At least 50 homes suffered damage in the southern state of Oaxaca, which, along with Mexico City, is still reeling from earthquakes that caused widespread damage in September.
The epicentre was around 145km from Pacific coast surfer resort Puerto Escondido in the southern state of Oaxaca and had a depth of 24.6km, according to the US Geological Survey.
At least two people died when a helicopter carrying Mexico's interior minister and the governor of Oaxaca crashed while trying to land after a tour of damage from the earthquake, officials said.
The senior officials survived.
The powerful, sustained shaking yesterday gave way to more than 225 aftershocks, the national seismology service said, and caused widespread panic.
In Mexico City, the seismic alarm sounded 72 seconds before tremors were felt, Mayor Miguel Angel Mancera said, giving residents time to flee to the streets.
Authorities said no deaths directly linked to the quake had been reported nationally.
The Oaxacan town of Jamiltepec appeared to sustain the heaviest impact in the southern region, with 50 homes damaged along with a church and government building, the state's civil protection agency said.
Patients were evacuated from a hospital there and from another in the nearby town of Putla Villa de Guerrero.
On a local highway, a fire ignited when two high-tension electric cables struck each other.
In the town of Pinotepa Nacional, close to the quake's epicentre, a photo obtained from Oaxaca's civil protection agency showed a single-story building where a portion of the brick facade had crumbled into the street.
A hospital was also damaged, and a collapsed structure blocked a major highway.
About 100,000 people in Oaxaca had lost power, the state's governor said.
National oil firm Pemex said its installations were in order, including its biggest refinery 386km from the epicentre.
Tremors were felt as far away as Guatemala to the south.
Images in the media appeared to show bricks and rubble fallen from buildings, and products tumbling off shelves in a supermarket.
In Mexico City, tall buildings swayed for more than a minute as seismic alarms sounded, with older structures in the chic Condesa neighborhood knocking into each other, and some cracks appearing in plaster and paintwork.
The Popocatepetl volcano south of the capital sent a kilometre-high column of ash into the sky, Mexico's disaster prevention agency said.
