MEXICO CITY (AFP) - A strong 6.4-magnitude earthquake rattled Mexico City on Thursday, causing buildings to sway, knocking out power and sending people fleeing into the streets.

There were no immediate reports of damage or injuries.

The US Geological Survey said the quake was centered 15 kilometers north of the town of Tecpan de Galeana in southern Guerrero state and 277 kilometers southwest of Mexico City.

The earthquake had a depth of 23 kilometers. The USGS initially measured it at 6.8-magnitude before revising it down.

"At first it felt like a jolt and then it started to move. We are used to it but you never know how long it will last or how strong it will be, which is why we always evacuate," said Daniel Rodriguez, 55, an engineer who fled his second-floor office.

As the earth shook, Finance Minister Luis Videgaray was delivering a speech and stopped mid sentence.

"If it's alright with you, we will take a break because it is shaking," Videgaray calmly told the audience.

Mexico's National Seismology Service reported at least two aftershocks of 5.1 and 4.9 magnitude.

The US Pacific Tsunami Warning Center said there was no widespread threat of destructive waves.

The temblor came three weeks after a powerful 7.2-magnitude earthquake shook the capital and the Pacific resort of Acapulco on April 18, breaking some windows and walls but sparing the country from any major damage or injuries.

Mexico City, a metropolis of 20 million people, is sensitive to distant earthquakes because it was built over soft soil from a drained lake that magnifies their effect.

In 1985, thousands of people were killed in Mexico City when buildings collapsed after an 8.1-magnitude temblor struck the Pacific coast.