MEXICO CITY — A 6.3-magnitude earthquake struck southwestern Mexico Monday, sending thousands of people into the streets of the capital in fear. There were no immediate reports of casualties or damage.

The quake struck at 1736 GMT with its epicenter located 158 kilometers (about 100 miles) from Huajuapan de Leon, in the state of Oaxaca, at a depth of 12.3 kilometers, the US Geological Survey said.

Residents said the quake rocked Mexico City, causing panicked people to flee houses and buildings in many areas of the capital. Thousands of people poured into the city's main downtown boulevard, Paseo de La Reforma.

The Seismological Institute of Mexico reported the earthquake was centered in the city of Pinotepa Nacional in Oaxaca and followed by at least seven aftershocks in the next two hours.

Mexico's interior secretary Alejandro Poire said on Twitter after contacting state authorities that there were "no reports of damage from the quake, but the assessments continue."

He was traveling with President Felipe Calderon during an official visit to Washington for a North American summit.

Mexico City's police chief, Manuel Mondragon, said an initial survey of the city from air and land had been completed "with no catastrophic situation to report."

Officials in Guerrero and Oaxaca states dispatched patrols to assess damage in rural areas near the epicenter, which already had been hit by a 7.4-magnitude quake on March 20, which killed two people and injured 13 others.

That earthquake -- with its epicenter south of the Pacific resort of Acapulco -- was the most powerful to hit the country since a deadly tremor in 1985, which destroyed entire neighborhoods of Mexico City and killed thousands.

More than 40 aftershocks, some measuring up to a magnitude of 5.0, shook the Mexican capital and southern areas in the aftermath of the March earthquake, according to Mexican seismologists.

People in Mexico's crowded capital -- with more than 20 million in the metro area -- are more than familiar with seismic activity and used to evacuation drills.