MEXICO CITY: At least 138 people died when a powerful earthquake of magnitude 7.1 struck central Mexico on Tuesday, toppling buildings in the heavily populated capital where rescuers scoured frantically under the rubble of ruins for survivors.

Thousands ran into the streets in panic, and millions lost electricity when the quake struck around lunchtime.

Mexico City Mayor Miguel Angel Mancera said 44 buildings were severely damaged or destroyed. Several major gas leaks and fires occurred.

Interior Secretary Miguel Angel Osorio Chong told local television rescue teams were working painstakingly with picks and shovels. The United Nations expressed condolences and said it was ready to assist.

"We have some buildings where we have reports that there could be people inside. They are doing it with lots of caution," the interior secretary said, adding that more rescue personnel would be needed.

Ambulances and fire engines confronted gridlock on Mexico City's streets as millions of workers tried to go home.

The temblor occurred on the 32nd anniversary of a devastating quake that killed thousands in Mexico City in 1985. Many Mexicans had taken part in earthquake drills on Tuesday as is customary every September 19.

A powerful quake in southern Mexico on September 7 killed at least 98 people.

Among buildings that collapsed in the capital on Tuesday were apartment blocks, a school, a factory and a supermarket. The fashionable Roma district was hard hit, and a six-story apartment building was among several collapses reported.

Hundreds of volunteers and rescue workers dug through the rubble with picks, shovels and their bare hands.

"My wife is there. I haven't been able to communicate with her. She is not answering, and now they are telling us we have to turn off our cellphones because there is a gas leak," said Juan Jesus Garcia, 33, choking back tears.

On Twitter, relatives posted pleas for news of family members, including 8-year-old Alexis Vargas Macias who was at Enrique Rebsamen school when the quake hit. The school collapsed, television images showed, and according to local media and family members on social media, several children were killed.

Earthquakes of magnitude 7 or above are regarded as major and are capable of causing widespread heavy damage.

The highest death toll was in Morelos State, just south of Mexico City, where 64 deaths were reported, said Luis Felipe Puente, head of the nation's civil protection agency. In Mexico City, at least 36 people were killed, he said.

In the neighboring state of Mexico, at least nine people were killed. Twenty-nine deaths were reported in the central state of Puebla, to the south, Puente said.

The epicenter of Tuesday's quake was in Puebla, the US Geological Survey said, and 41 deaths were reported there earlier. There was no explanation for the disparity with the figure Puente cited.

One death was also reported in the state of Guerrero, in southwestern Mexico, which Puente did not include in his count.

'FEAR AND PANIC'

Power was cut to 3.8 million customers, national electricity company CFE said.

In Puebla, university student Jevon Minto, 24, said he had just arrived at class when he felt the shaking. “We were seated when the place started shaking real, real hard ... You can literally feel the fear and the panic in this city.”

"You can see people ... (are) scared, their eyes are red from crying.”

US President Donald Trump said on Twitter: "God bless the people of Mexico City. We are with you and will be there for you." Mexico City, one of the world's most populous cities, and the surrounding area are home to about 20 million people.

In the capital, at least one survivor was pulled from a collapsed building in the busy Condesa neighborhood, and another was rescued from a six-story apartment building nearby.

Banker Jesus Gonzalez Hernandez, 55, said office lamps and furniture swayed when the tremor began. He and colleagues rushed to evacuate. "But while exiting down the stairs, the walls were coming apart,” said Gonzalez Hernandez, who fractured his ankle in the melee.

In Cuernavaca, a city in Morelos popular as a destination for weekend visitors from Mexico City, there were reports on local radio of people trapped beneath collapsed buildings.

Mexican TV and social media showed cars crushed by debris.

"We got out really fast, leaving everything as it was and just left," said Rosaura Suarez, as she stood with a crowd in Mexico City.

Mexican stocks and the peso currency dropped on news of the earthquake and Mexico's stock exchange suspended trading.

At the same time as the earthquake, Mexico's Popocatepetl volcano had a small eruption. In Atzitzihuacan on the slopes of the volcano, a church collapsed during mass, killing 15 people, Puebla Governor Jose Antonio Gali said.