Strong winds and rain from Hurricane Emily have begun lashing the Yucatan peninsula on Mexico's Caribbean coast.
Thousands have been forced to leave Cancun's resorts
Tourists in Cancun have abandoned the beaches and 15,000 employees have been forced to leave the Mexican state oil company's offshore platforms.
Emily, which experts say is extremely dangerous, passed 160km (100 miles) to the south of Jamaica but left at least four dead after rains caused flooding.
The Cayman Islands also felt the force of the storm early on Sunday.
Two pilots were killed in the Gulf of Mexico when fierce winds slammed their helicopter into the sea as they tried to airlift workers from an offshore oil rig.
Beachfront hotels in the popular resort of Cancun were boarded up as the hurricane, with winds of 215km/h (135mph), approached.
Strong winds were whipping sand from the beaches, toppling trees and bringing down billboards and power cables in resorts including Playa del Carmen, Tulum, the island of Cozumel and Cancun, reports said.
The US-based National Hurricane Center said Emily made landfall just north of Tulum at 0230 (0630 GMT), with the eye of the storm passing directly over Cozumel.
Some 30,000 tourists, the majority of them foreigners, have been moved inland to better-protected hotels or packed into emergency shelters in community centres and schools.
Cancun's international airport, earlier crammed with people waiting to leave, was closed down at about 1700 (2200 GMT) on Sunday.
"We're not going to sleep tonight," Cancun Mayor Francisco Alor said as Emily approached.
Local residents and fishermen were among those seeking refuge in makeshift emergency shelters.
"We live on a ranch about 10km (six miles) from here and I don't know if the roof is going to bear up. We left everything covered by tarpaulins," welder Ezequiel Martinez, 53, told Reuters news agency as he took refuge in Playa del Carmen.
Mexico's Tourism Secretary Gabriela Rodriguez earlier said some 85,000 people across Yucatan would be moved to safety in inland hotels, some as far away as Valladolid, the next province.
The country's state-owned oil company, Pemex, said production had been hit after it was forced to evacuate most of its offshore wells.
US experts say Emily is a category four hurricane, but it is so powerful that it may even become a rare category five like Hurricane Gilberto, which devastated the region in 1988.
Tourists and local residents were moved to emergency shelters
The storm, barrelling north-northwest at 30km/h (18mph), is expected to weaken slightly over northern Yucatan before heading into the Gulf.
From there, it could make landfall in north-eastern Mexico or southern Texas on Wednesday.
Jamaica was spared a direct hit by Emily, but the island still suffered flooding and landslides. About 70,000 households lost power and several homes and roads were washed away.
The bodies of a man, woman and two children were found inside a car swept over a cliff by flood water, Jamaican police said.
Emily comes less than a week after Hurricane Dennis caused more than two dozen deaths as it rampaged through Haiti, Jamaica, Cuba and Florida.
On Thursday Emily battered the eastern Caribbean island of Grenada, ripping off roofs and flooding streets.
Grenada is still recovering from Hurricane Ivan last year, which destroyed 90% of homes.