Holidaymakers are being warned devastating weather is on the way to a top tourist destination.
Forecasters say the monster Category 5 storm Hurricane Patricia, heading for Mexico's Central Pacific Coast, is the strongest ever recorded in the Western Hemisphere.
The area includes Puerto Vallarta, which is served by travel firm Thomson.
With maximum sustained winds near 200 mph, Patricia is the strongest storm ever recorded in the eastern Pacific or in the Atlantic, said Dave Roberts, a hurricane specialist at the US National Hurricane Centre.
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.
Officials in Mexico declared a state of emergency in dozens of municipalities in Colima, Nayarit and Jalisco states that contain the port of Manzanillo and the resort of Puerto Vallarta. The governor of Colima ordered schools to close on Friday, when the storm was forecast to make what the Hurricane Centre called a "potentially catastrophic landfall".
Pacific Coast residents boarded up homes and bought supplies ahead of the storm's arrival.
Rain pounded Manzanillo late on Thursday while people took last-minute measures ahead of Patricia, which quickly grew from a tropical storm into a Category 5 hurricane, leaving authorities scrambling to make people safe.
At a Wal-Mart store in Manzanillo, shoppers filled trolleys with non-perishable goods as a steady rain fell outside.
Veronica Cabrera, shopping with her young son, said Manzanillo tends to flood with many small streams overflowing their banks. She said she had taped her windows at home to prevent them from shattering.
Alejandra Rodriguez, shopping with her brother and mother, was buying 10 litres of milk, a large jug of water and items like tuna and canned ham that do not require refrigeration or cooking. The family has already blocked the bottoms of the doors at their home to prevent water from entering.
In Puerto Vallarta, restaurants and stores taped or boarded-up windows and residents raced to stores for last-minute purchases.
The Hurricane Centre in Miami warned that preparations should be rushed to completion, saying the storm could cause coastal flooding, destructive waves and flash floods.
"This is an extremely dangerous, potentially catastrophic hurricane," centre meteorologist Dennis Feltgen said.
Mr Feltgen said Patricia also poses problems for Texas. Forecast models indicate that after the storm breaks up over land, remnants of its tropical moisture will likely combine with and contribute to heavy rainfall that is already soaking Texas independently of the hurricane, he said.
"It's only going to make a bad situation worse," he added.
In Colima, authorities handed out sandbags to help residents protect their homes from flooding.
By early on Friday, Patricia's maximum sustained winds had increased to 200 mph - a Category 5 storm, the highest designation on the Saffir-Simpson scale used to quantify a hurricane's wind strength.
Patricia was centred about 160 miles south-west of Manzanillo early on Friday and was moving north-west at 10 mph on a projected track to come ashore between Manzanillo and Puerto Vallarta some time on Friday afternoon or evening.
Some fluctuations in intensity were forecast before then, but the Hurricane Centre said it is expected to be an "extremely dangerous" Category 5 storm.
A hurricane warning was in effect for the Mexican coast from San Blas to Punta San Telmo, a stretch that includes Manzanillo and Puerto Vallarta. A broader area is under hurricane watch, tropical storm warning or tropical storm watch.
The Hurricane Centre said Patricia is expected to bring rainfall of six to 12 inches, with isolated amounts of up to 20 inches in some locations.
Roberto Ramirez, the director of Mexico's National Water Commission, which includes the nation's meteorological service, said Patricia will be powerful enough to lift up cars, destroy homes that are not sturdily built with cement and steel and will be able to drag along people caught outside when the storm strikes.
Mr Ramirez said that the people in the most danger from the hurricane will be those on the coast, especially in the state of Jalisco.