People were forced to leave their homes and stand outside as high rise buildings swayed in Mexico City [Reuters]

A powerful 6.5-magnitude earthquake has struck Mexico's southwestern Guerrero state, shaking buildings and causing panic in the nation's capital and the Pacific resort of Acapulco.

The US Geological Service (USGS) initially estimated the quake at a magnitude of 6.8, but downgraded it to 6.7 and then 6.5.

Arturo Martinez, the state governer of Guerrero, told Al Jazeera that two people had died in the earthquake.

An 11-year-old boy was reported killed in Iguala city when a ceiling fell in on him and one man died in Mexico City, when a rock fell on his truck on the Acapulco highway.

The USGS said the quake was 64.9km deep and was centred about 42km southwest of Iguala in Guerrero, 166km south-southwest of Mexico City.

Authorities in Guerrero say they have found no structural damage and had no reports of injuries in the Pacific resort of Acapulco, which was about 140km from the quake's epicentre.

In Mexico City, high-rise buildings swayed for more than a minute, and customers were temporarily herded out of some shopping centres until the danger had passed.

Marcelo Ebrard, the capital's mayor, said on Twitter, the micro-blogging site, that no major damage had been reported.

He said power had failed in some parts of the city.

Parts of Mexico City rest on the shaky soil of a former lake bed, which tends to magnify the effect of earthquakes.

In 1985, an 8.1 magnitude earthquake killed as many as 10,000 people in the city.