The storm, which is a "Category 5," the highest rating possible, is expected to weaken somewhat before making landfall in the hurricane warning area by Friday afternoon or evening, the Miami-based hurricane center said.
"The hurricane is so strong that it could cross the country's two Sierra Madre mountain ranges, the two most mountainous regions, and come out the other side of the country along the Gulf of Mexico and head to the United States," said Roberto Ramirez, the head of federal water agency, Conagua.
Loudspeakers along the shore of the resort of Puerto Vallarta, popular with U.S. and Canadian tourists, blared orders to evacuate hotels as a light rain fell and a slight breeze ruffled palm trees. The streets emptied as police sirens wailed.
Hotel workers in Puerto Vallarta said efforts had begun to start evacuating guests, but others said they were still waiting to be told where to send them. When Reuters visited one of the city's designated shelters, a dilapidated-looking building in a low-lying area, there were still no evacuees to be found.
Aristoteles Sandoval, the governor of Jalisco state, expected 15,000 people to be evacuated from Puerto Vallarta.
Despite the looming mega-storm, by mid-morning a few people could still be seen swimming in the resort's long bay, and some chose to adopt a more philosophical outlook.
"It's natural to be worried, and then you breathe and it's gone," said Carolyn Songin, 52, a California resident visiting her friend Judith Roth, who owns a nearby yoga retreat.
Roth, a 66-year-old California native, said she would ride out the storm at Songin's "bunker-like" apartment.
"We're set up, we have our food and water, and we're just going to be in meditation and sending prayers for the area," Roth said.
The United States government issued an advisory urging its nationals to steer clear of beaches and rough seas, and to take shelter as instructed by Mexican officials.
Some businesses in Puerto Vallarta had begun boarding and taping up windows late on Thursday as a precaution, while several domestic flights had been delayed.
Mexican emergency officials began to prepare shelters and warned people in the states of Colima, Jalisco and Michoacan to get ready for torrential rainfalls.
Mexico shut its key cargo ports of Puerto Vallarta and Manzanillo on Friday.
Manzanillo is Mexico's busiest port for cargo.
None of the major installations of Mexican state oil giant Pemex lie in the projected path of the storm.
—CNBC contributed to this report.