Mexico City:  At least five people are dead after a strong magnitude 8.1 earthquake struck off the southern coast of Mexico, shaking buildings in the capital city and spurring tsunami warnings for waves up to three metres high.

People in Mexico City ran out into the streets after the quake struck at about 11.49pm, local time, as power went out in some neighbourhoods.

In the the suburb of La Roma, dozens of people stared at their phones in the dark to get news, while videos posted to social media showed power poles and monuments swaying as the earthquake struck.

At least three people in the southern state of Chiapas were killed, including two women who were crushed when buildings crumbled, said the state's governor Manuel Velasquez.

Two others - both children - died in the neighbouring state of Tabasco.

Tabasco governor Arturo Nunez said one of the children was killed by a falling wall, while the other was on a ventilator in hospital and died when the hospital lost power. 

The quake's epicentre was 87 kilometres south-west of the town of Pijijiapan, at a depth of 69.7 kilometres.

A warning for widespread, hazardous tsunami waves was issued by the Pacific Tsunami Warning Centre (PTWC).

Mexico, closest to the earthquake's epicentre, was expected to get tsunami waves of three metres or higher, with waves due to impact the coast from 12.40am local time (3.40pm AEST).

Hours after the quake, waves of 1 metre had been detected on the Mexican coast.

Tsunami waves reaching up to one metre above the tide level were possible for American Samoa, Antarctica, the Cook Islands, Ecuador, El Salvador, Fiji, French Polynesia, Guatemala, Kiribati, New Zealand, Samoa, Tokelau, Tuvalu, Vanuatu, and the islands of Wallis and Futuna, the PTWC said.

Waves of less than 30 centimetres above the tide line could impact some coasts of Australia, Chile, China, Colombia, Costa Rica, Guam, Hawaii, Honduras, Indonesia, Japan, Nauru, New Caledonia, Nicaragua, Panama, Papua New Guinea, Peru, the Philippines, Russia, Taiwan, Tonga and Vietnam.

"Hazardous tsunami waves from this earthquake are possible within the next three hours along some coasts," the PTWC said.

"Government agencies responsible for threatened coastal areas should take action to inform and instruct any coastal populations at risk in accordance with their own evaluation procedures and the level of threat.

"Persons located in threatened coastal areas should stay alert for information and follow instructions from national and local authorities."

As the waves began to impact, they were detected on coastal tide gauges.

The PTWC warned residents a tsunami is "a series of waves", and the time between wave crests could vary from five minutes to an hour.

"The hazard may persist for many hours or longer after the initial wave," the organisation's warning said.

Mexico's civil protection agency said the earthquake was the strongest to hit the country since a devastating 1985 tremor that toppled 400 buildings and killed thousands.

As many as 10,000 people were killed in that quake - measuring magnitude 8 - which resulted in Mexico City's construction codes becoming more strict.

This earthquake's magnitude was initially given as 8, before it was revised upwards by the US Geological Survey. The USGS reported multiple aftershocks, ranging in magnitude from 4.3 to 5.7.

The Pacific Tsunami Warning Centre measured the earthquake as being magnitude 8.2.

Luis Carlos Briceno, a 31-year-old architect who was visiting Mexico City, said he had never felt such a strong quake.

"I had never been anywhere where the earth moved so much," he said. "At first I laughed, but when the lights went out I didn't know what to do. I nearly fell over."

In neighbouring Guatemala, the president Jimmy Morales called for calm amid tsunami warnings.

The earthquake struck mainly in Guatemala's west, scattering bricks and glass on the ground in the state capital of Huehuetenango after walls collapsed.

Guatemala's second largest city Quetzaltenango, which was beginning to recover from a tremor in June, sustained more damage to its historic centre.

with Reuters, New York Times