Thousands of Mexicans are to be evacuated from their homes and tourists have been told to stay in their hotels as a hurricane sweeps along the coast.
The Mexican authorities have urged local people to take precautions
Hurricane John has weakened slightly to a category three storm, but is still sustaining winds of 205km/h (125mph).
It is moving north-west, parallel to the coastline, but so far the worst conditions have been at sea.
Heavy rains, sea surges and strong winds have been lashing some the country's coastal ports and beaches.
The move to evacuate residents follows an earlier hurricane warning along 400 miles (640km) of the Pacific coast, from the port of Lazaro Cardenas to San Blas. The popular resort of Puerto Vallarta is in the danger zone.
At least 10,000 people living in low-lying areas near the Los Cabos beach resort are to be evacuated by authorities before the storm hits on Friday.
Authorities have also ordered the evacuation of 8,000 people in the state of Jalisco and urged others to board up their homes as the hurricane drenches the coastline.
Earlier, the US National Hurricane Center (NHC) in Miami said Hurricane John was located about 110km (70 miles) south-west of the resort and port city of Manzanillo at 0200 local time (0900 GMT).
It was about 580km (365 miles) south of the tip of Baja California and moving at a speed of 22km/h (14mph).
It was expected to continue tracking west-north-west over the next 24 hours, the NHC said.
"On this track, John is expected to remain just offshore the Pacific coast of mainland Mexico. However, only a small deviation in the track could bring the centre onshore within the hurricane warning area," it said.
Rain has been falling along parts of the coastline and forecasters warn that heavy downpours of up to 35cm (1ft) could cause "life-threatening flash floods and mudslides".
Acapulco is already feeling the effects of Hurricane John
Acapulco has experienced sea surges of up to 10ft (3.5m), Reuters news agency reported.
Hurricane-force winds are approaching beaches south of Puerto Vallarta and the eye of the hurricane could pass close to the coast early on Thursday, the NHC said.
The hurricane's projected path takes it close to the southern tip of the Baja California peninsula before it moves out into Pacific waters.
The resort of Los Cabos, on the peninsula, is on alert as the storm draws nearer.
"We're watching it real closely. Everyone's taking it very seriously," said Minerva Saenz, who charters boats for tourists.
Hotel manager Jean-Louis Magron said guests had been told of the situation.
"If a hurricane comes, they are no longer guests - they are refugees," he said.
The Mexican army and emergency services are on alert along the coast.
Another storm, Kristy, has meanwhile formed in the Pacific but is expected to remain at sea, the hurricane centre said.