A gun battle between rival gangs in an overcrowded Mexican prison injured eight inmates, authorities said.

The shooting started Friday evening at the end of visiting hours as prisoners' family members and friends were leaving the Venustiano Carranza facility in Tepic, capital of the western state of Nayarit, state officials said.

Cora Cecilia Pinedo Alonso, Nayarit's interior secretary, said two gangs were grappling for control of drugs, weapons and other contraband openly bought and sold inside the prison.