People clear rubble after an earthquake hit Mexico City, Mexico, Sept 19, 2017. [Photo/Agencies]

Earlier in the day workplaces across the city held readiness drills on the anniversary of the 1985 quake, a magnitude 8.0 shake, which killed thousands of people and devastated large parts of Mexico City.

The new quake appears to be unrelated to the magnitude 8.1 temblor that hit Sept 7 off Mexico's southern coast and which also was felt strongly in the capital.

Much of Mexico City is built on former lakebed, and the soil can amplify the effects of earthquakes centered hundreds of miles away.

Mariana Morales, a 26-year-old nutritionist, 26, was one many who spontaneously participated in rescue efforts.

She wore a paper face mask and her hands were still dusty from having joined a rescue brigade to clear rubble from a building that fell in a cloud of dust before her eyes, about 15 minutes after the quake.

Morales said she was in a taxi when the quake struck, and she out and sat on a sidewalk to try to recover from the scare. Then, just a few yards away, the three-story building collapsed.

Gala Dluzhynska was taking a class with 11 other women on the second floor of a building on the trendy Alvaro Obregon street when the quake struck and window and ceiling panels fell as the building began to tear apart.

She said she fell in the stairs and people began to walk over her, before someone finally pulled her up.

"There were no stairs anymore. There were rocks," she said.

They reached the bottom only to find it barred. A security guard finally came and unlocked it.

AP/Xinhua