ACAPULCO, Mexico (Reuters) - Mexican authorities have found the remains of dozens of people in a series of clandestine graves in the restive southwestern state of Guerrero, which is racked by drug gangs, the state prosecutor's office said on Thursday.

The remains, including 32 corpses and nine severed heads, were found in a series of 20 hidden graves in the municipality of Zitlala, the prosecutor's office said in a statement.

The grisly discovery was made after residents reported an acrid smell.

The statement gave no details about how long the remains, which were being transferred to the state capital of Chilpancingo to be identified, had been buried. No suspects had been detained.

Roberto Alvarez, a spokesman for a security coordination group that includes members of Mexico's federal police, army and navy, said authorities would continue to search the area for more graves.

Cartels are battling over drug trafficking routes in Guerrero, which is one of the most violent states in Mexico.

(Reporting by Uriel Sanchez; Writing by Natalie Schachar; Editing by Simon Gardner and Paul Tait)