A powerful earthquake shuddered through Mexico City on Tuesday, destroying buildings and causing an unknown number of casualties among the megapolis' 20 million inhabitants, on the exact anniversary of a devastating 1985 temblor.
The US Geological Survey put the quake's magnitude at 7.1.
"I'm so worried. I can't stop crying. It's the same nightmare as in 1985," one resident in a plaza in the capital, Georgina Sanchez, 52, sobbed to AFP.
"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 quake -- which occurred in the early afternoon, hours after city authorities had conducted an earthquake drill -- caused most of its damage in the south of the sprawling city.
There and in the center, some buildings were reduced to debris and cars were flattened by falling stonework.
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 crumpled stone and tangled metal to seek survivors and bodies.
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 it happened he raced to the school where his children aged six and three were.
"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.
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.
In the central Roma district, a school had collapsed.
Witnesses said another school was smashed to rubble in Cuernavaca, a town just south of the capital.
Unconfirmed social media posts suggested the city's international airport had closed because of damage.
Mexico's Seismological Institute said the quake's epicenter was seven kilometers (four miles) west of Chiautla de Tapia, in the state of Puebla neighboring the Federal District where the capital is located.