A magnitude 8.1 earthquake off the coast of Mexico has sparked tsunami fears for much of Central America.

The monster earthquake was recorded off the coast of Chiapas, southern Mexico, at a depth of 69km about 5pm NZ time and was felt as far away as Austin, Texas, more than 2000 kilometres away. It was followed 30 minutes later by a magnitude 6 aftershock.

Mexican authorities said at least 35 people were confirmed dead, including many in the state of Oaxaca along the Pacific coast.

"The house moved like chewing gum and the light and internet went out momentarily," said Rodrigo Soberanes, who lives near the Chiapas state city of San Cristobal de las Casas.

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Soldiers remove debris from a partly collapsed municipal building felled by a massive earthquake in Juchitan, Oaxaca state, Mexico. Photo / AP
Soldiers remove debris from a partly collapsed municipal building felled by a massive earthquake in Juchitan, Oaxaca state, Mexico. Photo / AP

Mexican President Enrique Pena Nieto has spoken on national television, stating the quake was the strongest to be felt in the country for more than a century - and warning that aftershocks as high as magnitude 7 could follow in the next 24 hours.

"It was a large-scale earthquake," Mr Pena Nieto said. "It had a bigger magnitude than the one Mexicans knew in 1985."

Thousands were killed in four Mexican states in the 1985 event.

Shockwaves were felt in Mexico City, with buildings losing power and people running into the darkened streets in their nightwear shortly before midnight local time. The city was shaken for between four to six minutes.

Mexico City is over 750km from the epicentre in the southeastern state of Chiapas.

Pope Francis said he was prayingfor those killed and bereaved by the earthquake.

"I wish to express my spiritual closeness to all those suffering the consequences of the earthquake that struck Mexico last night ... My prayers for those who have lost their lives and their families," Francis said at a mass in Colombia.

EXTENSIVE CASUALTIES FEARED

Oaxaca state Governor Alejandro Murat told local news media that at least 23 people had died in his coastal state.

Civil defense officials said at least seven died in Chiapas and two others in the Gulf coast state of Tabasco.

The worst-hit city appeared to be Juchitan, on the narrow waist of Oaxaca known as the Isthmus. Video from the scene shows that about half the city hall collapsed in a pile of rubble and local officials said that at least 17 of the 32 dead were in Juchitan.
Officials said the death toll there could rise.

"There are houses that collapsed with people inside," Luis Felipe Puente, the agency's director general, told TV news channel Milenio.

Evacuated patients lie on their beds outside the hospital in the aftermath of a massive earthquake, in Juchitan, Oaxaca state, Mexico. Picture: AP
Evacuated patients lie on their beds outside the hospital in the aftermath of a massive earthquake, in Juchitan, Oaxaca state, Mexico. Picture: AP

Reports indicate a hospital and the town hall are among the collapsed structures.

Mexico's Governor for the state of Chiapas had earier reported three fatalities in San Cristobal de las Casas.

Tabasco state Governor Arturo Nunez confirmed the death of two children in his district.

He said that one died when a wall collapsed, and the other was a baby who died in a children's hospital that lost electricity, cutting off the supply to the infant's ventilator.

Automated damage estimate systems from the US Geological Survey (USGS) place the chance of fatalities from the shaking - not tsunami waves - in the coastal regions closest to the epicentre as being high. Projections based on population figures anticipate between 1000 and 10000 deaths.

EARLY RESPONSE SYSTEM SAVED LIVES

Sirens wailed across Mexico City warning its estimated 20 million population of a rapidly approaching quake from the Pacific, the latest use of its effective early warning system.

By the time alarmed residents felt the first shockwaves from the 8.1 magnitude quake, many had already made it onto the streets or into parks, well away from trembling and swaying buildings.

For the past two decades, Mexico's seismic alert system, known by the initials CIRES, has provided the city with an early warning of disaster, using a hundred sensors placed along its Pacific coast, where the risk of an earthquake is greatest.

It can take a minute for seismic waves from a quake's epicenter to reach the capital, several hundred kilometers away.

Even at that distance, Mexico City is vulnerable because it sits on an ancient lakebed and the relatively loose soil makes it prone to severe shaking.

Once the earthquake is detected, the system sets off a radio wave that triggers alarms in schools, ministries and offices, and automatically interrupts radio broadcasts.

Soldiers remove debris from a partly collapsed municipal building after an earthquake in Juchitan, Oaxaca state, Mexico. Photo / AP
Soldiers remove debris from a partly collapsed municipal building after an earthquake in Juchitan, Oaxaca state, Mexico. Photo / AP

WIDESPREAD DAMAGE

Economic losses from infrastructure damage is likely to be in the billions, it was reported.

Governor Manuel Velasco earlier told television station Televisa the rooves of homes and a shopping centre had collapsed in San Cristobal, Chiapas - a poor, largely indigenous state popular with tourists.

"There are damages in hospitals that have lost energy," he said. "Homes, schools and hospitals have been affected."

Authorities say that a hotel in Oaxaca has collapsed, but no one has been reported dead.

Civil Defence photos showed the crumbling facade of the Anel hotel in Matias Romero, which had split in half.

Mr Pena Nieto said no one was reported dead at the hotel.

Earlier, Governor Murat said that some people were able to escape from the hotel and authorities were working to determine if they were any casualties or missing people.

In neighbouring Guatemala, President Jimmy Morales spoke on national television to call for calm while emergency crews checked for damage.

Local radio in the Central American country reported one death, but it could not be confirmed. "We have reports of some damage and the death of one person, even though we still don't have exact details," Mr Morales said.

He said the possible death occurred in San Marcos state near the border with Mexico.

Mr Pena Nieto confirmed that major damage has been caused and that 1 million initially had been without power following the quake, but that electricity had been restored to 800,000 of them.

Debris from a collapsed wall sits in Oaxaca, Mexico. Picture: AP /Luis Alberto Cruz
Debris from a collapsed wall sits in Oaxaca, Mexico. Picture: AP /Luis Alberto Cruz

TSUNAMI WAVES DETECTED

The Pacific Tsunami Warning Centre says tsunami waves have been detected, but their full size and scale is yet to be determined.

Tsunami waves reaching 0.7 metres above the tide level were recorded by automated sea level gauges near the resort of Huatulco in Oaxaca state.

A wave of 1m in height has been recorded near Salina Cruz in Oaxaca state. Damage reports have yet to be made.

The centre earlier warned hazardous waves were possible, reaching as high as 3m above the tide level.

Mexico's National Seismological Service put the preliminary magnitude of the earthquake at 8.1, up from an initial estimate of 8.0. Initially the depth was believed to be 33km, but this has also been revised to 69km.

The USGS estimates 47 million people may have felt the quake, with one million experiencing 'very strong' shaking.

Lucy Jones, a seismologist in California who works with the US Geological Survey, said such as quake was to be expected.

"Off the west coast of Mexico is what's called the subduction zone, the Pacific Plate is moving under the Mexican peninsula," she said. "It's a very flat fault, so it's a place that has big earthquakes relatively often because of that.

"There's likely to be a small tsunami going to the southwest. It's not going to be coming up and affecting California or Hawaii," she said. "For tsunami generation, an 8 is relatively small."

- additional reporting AP