(CNN) At least 61 people have died after the most powerful earthquake to hit Mexico in a century struck off the southern coast.

The magnitude 8.1 quake , which was felt as far as Mexico City and Guatemala City, was registered off Mexico's southern coast just as heavy rains from Hurricane Katia lashed the east. The epicenter was in the Pacific Ocean, some 600 miles (1,000 kilometers) southeast of the capital and 74 miles (120 kilometers) off the coast.

President Enrique Peña Nieto said the temblor -- felt by about 50 million people across the country -- was the strongest earthquake Mexico has experienced in 100 years. In September 1985, a magnitude 8.0 earthquake killed an estimated 9,500 people in and around Mexico City.

This one hit late Thursday, when many people were asleep. The states of Chiapas and Oaxaca, home to about 9 million people, are located closest to the earthquake's epicenter. They are two of the most impoverished areas in Mexico, and were likely hit the hardest.

At least 45 people were killed in Oaxaca state, Civil Protection National Coordinator Luis Felipe Puente said on Twitter. Ten others died in Chiapas state and three were killed in Tabasco, he said.

Latest updates

-- A red alert was issued by the US Geological Survey (USGS) PAGER system , which predicts economic and human loss after earthquakes. "High casualties and extensive damage are probable and the disaster is likely widespread. Past red alerts have required a national or international response," it said.

-- The USGS reported multiple aftershocks, including at least six with tremors measuring above 5.0 in magnitude.

-- The quake had a depth of 69.7 kilometers (43 miles), according to the USGS, which makes it particularly shallow, said Jana Pursely, a staff geophysicist. That means more intense shaking.

-- A tsunami was confirmed in Mexico, with one wave coming in at 3 feet (1 meter), according to a tweet from the National Weather Service's Pacific Tsunami Warning Center. Tsunami waves taller than 10 feet (3 meters) could hit the coast of Mexico, while 3-foot waves could reach as far as Ecuador, New Zealand and Vanuatu, it said.

-- Mexico's army, marines and federal police were mobilized to respond, Peña Nieto said.

-- About 1.85 million homes lost electricity, but 74% of them have had service returned, Peña Nieto said. Some people lack water service, and it may take 36 to 48 hours to get it back up and running.

-- Toppled buildings could be seen on video shared on social media from the city of Juchitan in Oaxaca state, where a local councilwoman said co-workers were trapped in the municipal building. A hospital there collapsed, and patients were seen on another video receiving treatment in an open field.

-- Four people may be trapped inside a collapsed hotel in Oaxaca, Oaxaca Civil Protection Director Amado Bohorquez told CNN.

Photos: Deadly quake strikes off Mexican coast Soldiers remove debris from a partially collapsed municipal building in Juchitan, Mexico, on Friday, September 8. A magnitude-8.1 earthquake was registered the night before off Mexico's southern coast. It is the strongest quake to hit the country in 100 years, according to President Enrique Peña Nieto. Hide Caption 1 of 12 Photos: Deadly quake strikes off Mexican coast People stand on a building's rubble in Juchitan on September 8. Hide Caption 2 of 12 Photos: Deadly quake strikes off Mexican coast Residents of Tapachula, Mexico, stay in a shelter after the quake. Hide Caption 3 of 12 Photos: Deadly quake strikes off Mexican coast Men survey damage in Veracruz, Mexico. Hide Caption 4 of 12 Photos: Deadly quake strikes off Mexican coast Medical staff and patients wait outside after a hospital was evacuated in Villahermosa, Mexico. Hide Caption 5 of 12 Photos: Deadly quake strikes off Mexican coast This photo shows a collapsed building in Matias Romero, Mexico, early on September 8. Hide Caption 6 of 12 Photos: Deadly quake strikes off Mexican coast People gather on a street in downtown Mexico City on September 7. Hide Caption 7 of 12 Photos: Deadly quake strikes off Mexican coast People gather outside a Mexico City nightclub after the quake. Hide Caption 8 of 12 Photos: Deadly quake strikes off Mexican coast A woman gestures toward her earthquake-damaged home in Coatzacoalcos, Mexico. Hide Caption 9 of 12 Photos: Deadly quake strikes off Mexican coast People sit on a sidewalk in Mexico City after the quake. Hide Caption 10 of 12 Photos: Deadly quake strikes off Mexican coast Damage is seen inside a mall in Tuxtla Gutierrez, Mexico. Hide Caption 11 of 12 Photos: Deadly quake strikes off Mexican coast People react after the quake jolted the Mexico City International Airport. Hide Caption 12 of 12

Relief efforts

Hurricane Katia will complicate the delivery of relief supplies to the hard-hit regions, Eduardo Mendoza, general manager of Direct Relief Mexico , told CNN on Friday. Heavy rains from the storm could also contribute to water-borne illnesses, he said.

Large trucks are having a difficult time reaching affected areas, he said, so individuals are now bringing in supplies in their personal cars.

Mendoza said Direct Relief coordinates the delivery of supplies from companies in Mexico and the United States.

"What they really need right now are basic medical supplies for wound care and other trauma care," he said.

Also needing help are people who fled their residences and left their medications behind.

Mendoza said he felt the earthquake in his Mexico City residence.

"I could see my wall moving half a foot," he said. He ran outside but the building kept shaking 30 or 40 more seconds, he said.

Chiapas hit hard

Gonazalo Segundo was awakened by the shaking.

"I was already in bed. I was in my place, so we were expecting to have a tranquil night but suddenly ... everything breaks apart, glasses, furniture and everything," he told CNN by phone from Chiapas.

First responders survey the scene after the quake.

"We have experienced earthquakes before, but not like this. It was so intense," Segundo said. "We are alive, that's the important thing."

Pursely, of the USGS, told CNN she expects damage along the coast, meaning a costly cleanup could be on the way, adding that these types of shallow quakes have the potential to be very dangerous.

In Tuxtla Gutierrez, the capital of Chiapas, Mexico's poorest state , Roberto Cardenas said he was watching television late Thursday when the house began shaking violently.

"The shaking lasted more than three minutes," he said. "I felt desperate and we got my family out of the house. We got outside and the ground was still shaking."

Cardenas said he remained outside for hours with family members and neighbors. They feared strong aftershocks but were forced back home by heavy rains about 5 a.m.

Many older structures in the state capital were damaged but his home escaped largely unscathed, Cardenas said.

Chiapas Gov. Manuel Velasco told Foro TV that there have been reports of damage, including hospitals that have lost power and buildings with collapsed roofs.

People gather on a street in downtown Mexico City during the quake.

Mexico City shakes

Videos on social media showed significant tremors in various parts of the country, as well as major damage to buildings and infrastructure. Traffic lights could be seen shaking, videos showed.

El sismo de hoy .. el más fuerte de los últimos 30 años. La alarma sísmica !! Al 100% 👏🏼👏🏼... todos bien !??? pic.twitter.com/ZXrxvcjogg — Hussein Forzán ® (@husseinforzan) September 8, 2017

Mexico's capital, hundreds of miles away, was not spared the quake's tremors. Parts of the city lost power, Mayor Miguel Angel Mancera said in an interview on Foro TV.

Paulina Gomez-Wulschner was driving in Mexico City when the quake struck. She heard an earthquake alarm go off on the radio, parked her car and joined others who stood in the middle of the street to avoid falling objects.

"This was a very, very strong earthquake, one of the strongest I've felt, and I was here in 1985 when that earthquake collapsed Mexico City," she told CNN. "It was very scary."

Gomez-Wulschner said she could hear sirens, ambulances and helicopters in the aftermath but did not see any immediate damage near her.