Hurrican Patricia has reached Category 5, the highest, and was heading towards southwest Mexico early Friday.
The centre of the storm was around 320 kilometres south-southwest of the city of Manzanillo at 0300 GMT, said the National Weather Centre in the United States.
Patricia had windspeeds of up to 260 kilometres per hour.
The storm, moving north-northwest at 17 kilometres per hour was predicted to make landfall overnight.
The Mexican government declared a state of emergency in 56 localities in coastal areas.
Fishing crews were told to remain in harbour. People were advised against tourism or watersports on the coast.
A few airlines cancelled flights to the resort of Puerto Vallarta.
In Guerrero, Michoacan, Colima and Jalisco states strong rain and waves up to 4 metres high were expected.
Authorities warned of floods and landslides.