A powerful earthquake shook Mexico City on Tuesday, causing panic among the megalopolis' 20 million inhabitants on the 32nd anniversary of a devastating 1985 quake.

The US Geological Survey put the quake's magnitude at 7.1 while Mexico's Seismological Institute said it measured 6.8 on its scale.

The institute said the quake's epicenter was seven kilometers west of Chiautla de Tapia, in the neighboring state of Puebla.