At least 138 people were killed when a powerful, 7.1-magnitude earthquake rocked Mexico on Tuesday (September 19^th), toppling buildings in the capital and sowing panic on the anniversary of a devastating 1985 quake.

People remove debris of a building which collapsed after a quake rattled Mexico City. (Omar TORRES/AFP)

The toll from the authorities was preliminary and could rise, as rescue crews and volunteers in Mexico City - home to 20 million people - clawed through the rubble of collapsed buildings looking for survivors and bodies.

Four deaths were initially confirmed in Mexico City. The others occurred in nearby regions, mostly in Morelos state just to the south where 42 deaths were recorded.

"I'm so worried. I can't stop crying. It's the same nightmare as in 1985," Georgina Sanchez, 52, sobbed to AFP in a plaza in the capital.

"We ran outside thinking all was going to collapse around us," said Lazaro Frutis, a 45-year-old who escaped an office building before it crumpled to the ground. "The worst thing is, we don't know about our families or anything."

The U.S. Geological Survey said the quake's epicentre was seven kilometres west of Chiautla de Tapia, in the neighbouring state of Puebla. (Source: U.S. Geological Survey)

The quake - which occurred in the early afternoon, hours after city authorities had conducted an earthquake drill - caused damage in the bustling center of the city, and to areas south and west of the capital.

Several buildings were reduced to debris and cars were flattened by falling masonry.

Scenes of chaos permeated the city, with traffic jammed to a standstill before blanked-out stop lights, and anxious people running between vehicles as ambulances tried to make headway, sirens squealing.

Emergency officials warned people in the streets to avoid smoking because of the risk of igniting gas leaking from ruptured pipes.

In several locations, people were seen clambering on buildings that were now piles of stone and tangled metal to seek pull people out.

'EVERYONE WAS FRANTIC'

The disaster immediately recalled the 1985 quake in which more than 10,000 people died, escalating panic among the population.

Jorge Lopez, a 49-year-old Spaniard living in Mexico City, said that when the quake happened he raced to the school in the central Roma district where his children aged six and three were, to find it collapsed but his offspring safe if terrified.

"We arrived at the school and everyone was crying, everyone was frantic, and the kids were holding on to a rope," he said. "It's uncontrollable. You can't do anything against nature," he said.

Witnesses said another school was smashed to rubble in Cuernavaca, a town just south of the capital. The fate of the pupils and teachers was unknown.

An office building of around five stories in the chic Condesa district of central Mexico City had collapsed. Volunteers scrambled among the debris, pulling out three survivors and looking for more. "There are people trapped there!" yelled one woman.

Similar efforts were made at other smashed buildings nearby. At one, an emergency worker held up a sign commanding "Silence" so crews could listen for the sounds of any survivors.

Patients were evacuated from a hospital in the adjoining Roma district, wheeled out on beds and wheelchairs as staff set up makeshift wards outside.

Picture of a car smashed by debris from a damaged building after a powerful earthquake rattled Mexico City. (Alfredo ESTRELLA/AFP)

Mexico's President, Enrique Pena Neto, said on Twitter he had ordered the evacuation of damaged hospitals "and the transfer of their patients to other medical facilities." He was to hold an emergency coordination meeting after overflying the disaster zone.

At one collapsed building in Roma, dozens of people clawed at the rubble as they waited for the arrival of heavy machinery to move the heavy chunks of stone. Officials called out for more volunteers, and for water.

A woman standing and watching the efforts with her husband, a doctor, turned to him and said, "Darling, if you want to help, give me your glasses and take care."

Hours after the quake, residents stood around outside, in the streets, fearing aftershocks.

Unconfirmed social media posts suggested the city's international airport had closed because of damage.

Mexico's stock market was shut because of the quake.

PRESIDENT TRUMP'S PRAYERS

Officials in other countries began to react to the disaster, with many offering to help.

"With rising casualties & many collapsed buildings, my thoughts are with those impacted by the Mexico earthquake. The U.N. is ready to support," tweeted the U.N.'s emergency relief coordinator, Mark Lowcock.

Costa Rican President Luis Guillermo Solis, who was in New York with other world leaders for a U.N. General Assembly, expressed his "solidarity" with the Mexican people.

U.S. President Donald Trump, who has forged an antagonistic relationship with Mexico since coming to office, tweeted: "God bless the people of Mexico City. We are with you and will be there for you."

Justin Trudeau, the Prime Minister of Canada, tweeted: "Devastating news from Mexico City. My thoughts are with those affected by today's earthquake - Canada will be ready to help our friends."

Mexican Foreign Minister Luis Videgaray responded by saying "Mexico sincerely thanks the displays of international solidarity that we are receiving."

VNF/CNA