(Adds revision of magnitude, no reports of damage)
MEXICO CITY, Aug 21 (Reuters) - A magnitude 6.1 earthquake shook Mexico City on Wednesday, its epicenter located in the poor southern state of Guerrero, the United States Geological Survey (USGS) said, but there were no immediate reports of damage or injuries.
Buildings swayed for around a minute in Mexico City, and several offices evacuated staff.
The epicenter was 11 miles (18 km) northwest of the town of Ayutla de los Libres, the USGS said, inland from Mexico's Pacific coast and the popular tourist resort of Acapulco.
It said the tremor struck at a depth of 21.6 miles (34.8 km). The USGS initially reported the quake at 6.4 magnitude, but then revised down the reading.
Mexico City Mayor Miguel Angel Mancera said on Twitter that there were no immediate reports of injuries or damage in the capital. (Reporting by David Alire Garcia; Editing by Simon Gardner and David Brunnstrom)