The storm was "almost stalled off the coast" which could make it more dangerous since that could increase the volume of rain, said National Weather Service chief Alberto Hernandez.
"It is already generating a huge amount of rain" in Guerrero state, he added. Authorities there scrambled to get students from primary to university level home to safety.
At 2100 GMT yesterday, Marty was packing top sustained winds of 130 kilometers per hour, and was located 145 miles west of Acapulco, the US National Hurricane Center said.
In either case, the hurricane will create driving rains, they warned.
Two years ago, mountainous Guerrero state was simultaneously hammered by hurricanes Ingrid and Manuel, killing 157 people.