MEXICO CITY (AFP) - Tuesday's destruction revived horrific memories in Mexico on the anniversary of another massive quake in 1985, the disaster-prone country's deadliest ever.

One of the most gut-wrenching scenes was at the Enrique Rebsamen primary school on Mexico City's south side, whose three floors collapsed into one, trapping students and teachers inside.

"We have a report of 25 dead, among them 21 children and four adults" at the Enrique Rebsamen elementary school, Mexican Undersecretary of Education Javier Trevino told the Televisa network.

President Enrique Pena Nieto, who rushed to the site, warned the death toll could rise.

"There are 30 children and eight adults still missing. Rescue operations are ongoing," he told journalists.

Local media reports said soldiers had located one child alive beneath the ruins. They administered oxygen through a tube, but were so far unable to extract him.

Local media reported that families were getting WhatsApp messages pleading for help from desperate relatives trapped under debris.

Memories of the devastating 1985 earthquake, which killed at least 10,000 people, surged to the surface on what was meant to be a low-key 32nd anniversary.

Adding to the national sense of vulnerability, the quake also came just 12 days after another that killed nearly 100 people and left more than 200 injured, mainly in the southern states of Oaxaca and Chiapas.

Many in the capital quickly ran for safety outdoors when walls around them swayed and cracked. "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.

The quake - which occurred in the early afternoon, hours after city authorities had conducted an earthquake drill - caused massive damage in the bustling center of the city.

"It was horrible," said resident Leiza Visaj Herrera, 27. "I had to hold on to the ground."

After the screams and the shock, people quickly set to work digging for survivors.

Central areas like Roma, Condesa and Doctores appeared to have taken the brunt of the 7.1-magnitude quake.

Amid the tears and terror, there were small victories.

Cheers and applause echoed around the crowded Calle Alvaro Obregon in Roma as rescuers managed to pull someone from under the rubble.

There remained a sense that such moments would be rare, however.

In other buildings in the same area, volunteers joined the authorities to remove debris.

Scenes of chaos erupted in the quake's aftermath.

Traffic jammed to a standstill before blanked-out stop lights, and anxious people ran between vehicles as ambulances tried to make headway, sirens blaring.

In several locations, large crowds of people clambered on buildings that were now piles of stone and tangled metal, trying to pull people out.

Emergency workers held up signs commanding "Silence" so crews could listen for the sounds of any survivors.

Jorge Lopez, a 49-year-old Spaniard living in Mexico City, said he raced to his children's school in the central Roma district, to find it collapsed but his offspring safe but terrified.

"We arrived at the school and everyone was crying, everyone was frantic, and the kids were holding on to a rope," he said.

Patients were evacuated from a nearby hospital, wheeled out on beds and wheelchairs.

Pena Neto said on Twitter he had ordered the evacuation of damaged hospitals.

At one collapsed building in the Roma district, dozens of people dug through rubble as they waited for the arrival of heavy machinery to move the massive 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, go ahead. Just give me your glasses, and be careful."

Mexico City's international airport closed for more than three hours following the quake.

The stock market was forced to shut.

Fearful residents whose homes were damaged were preparing to spend the night on the street or in parks.

On the clogged and darkened roads, muggers came out at night to assault motorists.

Officials in several other countries responded to the quake with offers of help.

Honduras sent a 36-strong rescue team.

US President Donald Trump, who has forged an antagonistic relationship with Mexico, 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."