A strong 6.4-magnitude earthquake rattled Mexico on Thursday, causing a bridge to collapse in the south and sending people fleeing into the capital's streets as buildings swayed.

Mexico City, a megalopolis of 20 million people, escaped major damage and injuries as residents evacuated homes and offices.

Hundreds of tourists calmly left their hotels in the Pacific resort of Acapulco and returned after a few minutes.

But a large chunk of a road's bridge collapsed in Tecpan de Galeana, near the epicenter of the earthquake in southern Guerrero state, according to a picture posted by the state's public safety agency on Twitter.

Guerrero state authorities were reviewing the region's infrastructure for any damage. None was reported in the state capital, Chilpancingo.

The US Geological Survey said the quake was centered 15 kilometers (eight miles) north of Tecpan de Galeana and 277 kilometers (172 miles) southwest of Mexico City.

The earthquake had a depth of 23 kilometers. The USGS initially measured it at a magnitude of 6.8 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 Mexico City 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 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.