Witnesses have described their horror after the most powerful earthquake to hit Mexico in more than eight decades destroyed more than 1500 buildings and killed at least 58 people.

The magnitude-8.1 earthquake shook Mexico’s capital city just before midnight (3pm Friday AEST), causing people to flee into the streets, knocking out electricity to 800,000 homes and unleashing a series of small tsunamis across the Pacific.

One Mexico City resident sat vigil with a relative by the body of a loved one draped in a red shroud.

“Well, we haven’t been able to do anything,” Alma Rosa said.

“We are holding vigil for her here because we went to purchase her coffin but there are none left because of how many people were killed.

“We couldn’t find a coffin and they haven’t come by to drop one off for us.”

Windows were broken at Mexico City airport and up to 1 million people were without power in several neighbourhoods of the capital as alarms sounded and pyjama-clad residents ran into the streets of the city after the quake struck, a Reuters witness said.

Epicentre, aftershocks and tsunamis

The epicentre of the latest quake was 123km southwest of the town of Pijijiapan in the southern state of Chiapas, at a revised depth of 69km, the US Geological Survey says.

USGS reported six aftershocks, ranging in magnitude from 4.5 to 5.7.

Some 62 aftershocks followed the quake, with seismologists saying it is possible the next 24 hours could see a further major aftershock of as much as 7.2.

The US Tsunami Warning System said hazardous tsunami waves were possible on the Pacific coasts of several Central American countries, including Guatemala, El Salvador, Costa Rica, Nicaragua, Panama and Honduras.

A tsunami was also confirmed for Mexico, with the largest wave measuring one metre, according to the Pacific Tsunami Warning Centre.

Townships in ruins

In Juchitan, shocked residents stepped through the rubble of dozens of collapsed buildings.

“It was all horrific. Everything collapsed, everything,” Maria Magdalena Lopez said, in tears outside its shattered walls.

“The truth I have no words to explain what happened. Look at my home, everything is destroyed.”

In another neighbourhood, dozens stood outside, some wrapped in blankets against the cool night air. Children were crying.

Civil Defence Forces members help residents in Tapachula, Mexico, as a state of emergency is declared in some parts of the country. Photo: EPA/Jose Torres

Liliana Villa, 35, who was in her apartment when the quake struck, fled in her nightclothes.

“It felt horrible, and I thought, ‘this [building] is going to fall,'” she said.

Matias Romero resident Rosa Esteva Luis said when she arrived at her mother’s house, “she was crying”.

“And my neighbour had the ceiling fall on top of his head, I don’t know if he is alive,” she said.

Residents remain holed in up shelters as the death toll mounts. Photo: EPA/Jose Torres

Three people were killed in Chiapas and two more are known to have died in neighbouring Tabasco state.

Hundreds of buildings were severely damaged in the south of the country, including a hotel where rescuers were searching to make sure nobody remained trapped in the rubble.

Worst quake in 100 years

Mexico’s President Enrique Pena Nieto described the earthquake as “the biggest magnitude to be recorded in our country in at least the last 100 years”, rating it at 8.2, slightly more powerful than the measurement provided by US seismologists.

The quake was apparently stronger than a devastating 1985 tremor that flattened swathes of Mexico City and killed thousands.

Authorities attributed the lower cost in human life to stronger building codes introduced after the 1985 catastrophe.

— with ABC/wires