Hurricane Patricia is powering towards south-western Mexico as a monster category five storm, the strongest ever in the Western Hemisphere.
Residents and tourists are hunkering down or trying to make last-minute escapes ahead of what forecasters called a "potentially catastrophic landfall" later on Friday.
The storm is homing in on a Pacific coastline dotted with sleepy fishing villages and gleaming resorts, including the popular beach city of Puerto Vallarta and the port of Manzanillo.
After hitting land, Patricia's projected path will quickly take it over mountainous terrain that is prone to dangerous flash floods and landslides.
In Puerto Vallarta, residents have reinforced homes with sandbags and shop windows with boards and tape, and hotels rolled up beach-front restaurants.
The airport is closed to all flights and all but deserted, but queues formed at a bus station with people anxious to buy tickets to Guadalajara and other inland destinations.
Fire service vehicles and ambulances rolled through the streets, sirens blaring, as emergency workers warned people in both Spanish and English to evacuate.
Jose Manuel Gonzalez Ochoa, whose family lives in their ground-floor restaurant, said neighbours told them water was 5ft deep in the street the last time a hurricane hit.
He said his family is heading to a town 30 minutes from the coast. "We're better off heading up there already," he said. "The whole government is telling us to leave. You have to obey."
Asked what preparations he had made for his business, he said he would just close it up and see what is left after the storm passes.
Patricia formed suddenly on Tuesday evening as a tropical storm, turned into a hurricane just over a day later and kept building in strength, catching many off guard with its rapid growth.
By Friday it was the most powerful hurricane on record in the Western Hemisphere, with maximum sustained winds near 200mph, according to the US National Hurricane Centre in Miami.
Patricia's power is comparable to that of Typhoon Haiyan, which left more than 7,300 dead or missing in the Philippines two years ago, according to the UN's World Meteorological Organisation.
By late Friday morning, Patricia's centre was about 125 miles south-west of Manzanillo, and about 195 miles south of Cabo Corrientes.
The Hurricane Centre said Patricia is expected to remain an "extremely dangerous" category five hurricane through landfall late afternoon or evening local time, before weakening over the inland mountains.
Mexican officials have declared a state of emergency in dozens of municipalities in Colima, Nayarit and Jalisco states, and schools are closed.
The government suspended tolls on the Guadalajara-Tepic highway to facilitate the flow of vehicles from the coast.
According to the 2010 census, there were more than 7.3 million inhabitants in Jalisco state and more than 255,000 in Puerto Vallarta municipality. There were more than 650,000 in Colima state, and more than 161,000 in Manzanillo.
Roberto Ramirez, director of Mexico's National Water Commission, which includes the nation's meteorological service, said Patricia's winds will be powerful enough to lift cars, destroy homes that are not well-built and drag anyone caught outside away.
US State Department spokesman Mark Toner said American authorities are closely monitoring the path of Patricia and its potential impact on the US citizens who live in the affected area.
He said tens of thousands of Americans are believed to live or be on holiday in the area.
US officials are closely co-ordinating with Mexican authorities, he said, and are advising American citizens to follow guidance from local authorities.
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