A 7.6 magnitude earthquake shook Mexico Tuesday

A 7.6 magnitude earthquake shook southern Mexico and the capital on Tuesday, in what witnesses described as "long" and "strong," according to the Associated Press.

There were no immediate reports of major damage or casualties but many workers left office buildings in Mexico City as a precautionary measure.

The AP later reported that the earthquake had been downgraded to one of 7.4 magnitude, and was followed by an aftershock that measured 5.1 in magnitude and was felt in the capital.

A pedestrian bridge collapsed in Mexico City and a building in the neighborhood of Condesa looked on the verge of collapse, but there were no reports of deaths.

President Felipe Calderon posted a message on Twitter just minutes after the quake that no major damage was reported, Bloomberg reported.

The mayor of Mexico City, Marcelo Ebrard, surveyed the city by helicopter and reported that he did not see any destruction.

The epicenter of the earthquake is said to be in a rural area between the southern city of Oaxaca and the coastal resort of Acapulco.

Ometepec, Guerrero is only 15 miles from the epicenter, and experienced many aftershocks. The city's comptroller, Francisca Villalva Davila, told CNN, "There are many cracked ceilings, many houses that collapsed."

According to Reuters, the Pacific Tsunami Warning Center stated that a destructive tsunami was unlikely but that high waves were possible.

It is the strongest earthquake to hit Mexico since a 7.8 magnitude quake in 1985, which killed thousands.

The quake, which struck at noon (CST), was felt as far south as Chile.