By David Alire Garcia and Jose Cortes

JUCHITAN, Mexico (Reuters) - At least 60 people died when the most powerful earthquake to hit Mexico in over eight decades tore through buildings and forced mass evacuations in the poor southern states of Oaxaca and Chiapas, triggering alerts as far away as Southeast Asia.

The 8.1 magnitude quake off the southern coast late Thursday was stronger than a devastating 1985 temblor that flattened swathes of Mexico City and killed thousands.

This time damage to the city was limited as the quake was deeper and farther away, but it still sent thousands of people scurrying from their homes onto the streets when the violent rumbling began that also shook Guatemala and El Salvador.

"It almost knocked me over," said Gildardo Arenas Rios, a 64-year-old security guard in Mexico City's Juarez neighborhood, who was making his rounds when buildings started to tremble.

The Oaxacan town of Juchitan on Mexico's narrowest point bore the brunt of the disaster, with sections of the town hall, a hotel, a church, a bar and other buildings reduced to rubble.

"The situation is Juchitan is critical; this is the most terrible moment in its history," the local mayor, Gloria Sanchez, said a few hours before President Enrique Pena Nieto flew to the battered town to oversee rescue efforts.

Facades of shattered buildings, fallen tiles and broken glass from shop fronts and banks littered the pavements of Juchitan while heavily armed soldiers patrolled and stood guard at areas cordoned off due to the extent of the damage.

Startled residents stepped through the rubble of about 100 wrecked buildings, including houses, a flattened Volkswagen dealership and Juchitan's shattered town hall. Scores paced the terrain or sat outside warily, mindful of the frequent aftershocks.

"Look at what it did to my house," said Maria Magdalena Lopez, in tears outside its battered walls. "It was horrifying, it fell down."

Alma Rosa, sitting in vigil with a relative by the body of a loved one draped in a red shroud, said: "We went to buy a coffin, but there aren't any because there are so many bodies."

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All the deaths were in three neighboring states clustered near the epicenter that lay about 70 km (40 miles) off the coast.

In Oaxaca, 45 people died, many of them in Juchitan, while in Chiapas 12 and in Tabasco three people lost their lives, according to federal and state officials.

In Chiapas, home to many of Mexico's indigenous ethnic groups, thousands of people in coastal areas were evacuated as a precaution when the quake sparked tsunami warnings.

Waves rose as high as 2.3 feet (0.7 meter) in Mexico, the Pacific Tsunami Warning Center said, though that threat passed.

State oil company Pemex said it was checking its installations for damage and closed the Salina Cruz refinery in the same region as the epicenter as a precautionary measure. It began restarting the 330,000 barrel-per-day refinery on Friday afternoon.

WOKEN IN THE NIGHT

At least 250 people in Oaxaca were also injured, according to agriculture minister Jose Calzada.

Classes were suspended in much of central and southern Mexico on Friday to allow authorities to assess the impact. Dozens of schools were damaged, officials said.

People ran into the streets in Mexico City, one of the world's largest metropolises and home to more than 20 million, and alarms sounded after the quake struck just before midnight.

Scores stood outside in central neighborhoods, some wrapped in blankets against the cool night air. Children were crying.

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.

The U.S. Geological Survey (USGS) said the quake's epicenter was 54 miles (87 km) southwest of the town of Pijijiapan at a depth of 43 miles (69 km).

John Bellini, a geophysicist at the USGS National Earthquake Information Center in Golden, Colorado, said Thursday's quake was the strongest in Mexico since an 8.1 temblor struck the western state of Jalisco in 1932.

Across the Pacific, the national disaster agency of the Philippines put the country's eastern seaboard on alert for possible tsunamis, although no evacuations were ordered.

OUTAGES, AFTERSHOCKS

Rescue workers searched through the night for anyone trapped in collapsed buildings, but the toll appeared to be less severe than that seen in some far less powerful tremors.

Windows were shattered at Mexico City airport and power went out in several neighborhoods of the capital, affecting more than 1 million people. The cornice of a hotel came down in the southern tourist city of Oaxaca, a witness said.

Mexio City is built on a spongy, drained lake bed that amplifies earthquakes along the volcanic country's multiple seismic fault lines, even when they occur hundreds of miles away.

The 1985 earthquake was just inland, about 230 miles from Mexico City. Thursday's quake was 470 miles from the city.

Authorities reported dozens of aftershocks, and President Pena Nieto said the quake was felt by around 50 million of Mexico's roughly 120 million population.

Mexico is evaluating whether the quake will trigger a payout from a World Bank-backed catastrophe bond, Finance Minister Jose Antonio Meade said on Friday. Meade said the bond's coverage could reach $150 million, depending on magnitude and location.

But he said Mexico has sufficient funds to pay for a cleanup whether the bond was triggered or not.

(Reporting by Mexico City Newsroom; Writing by Dave Graham; Editing by Bernadette Baum, James Dalgleish and Jonathan Oatis)