MIAMI, Florida (CNN) -- Hurricane Gustav churned into the Gulf of Mexico on Saturday night, still an "extremely dangerous" Category 4 storm threatening to blast the same region devastated by Hurricane Katrina three years ago.
Gustav, a Category 4 storm with 135 mph (220 km/hr) top winds, was centered 485 miles (780 kilometers) southeast of the mouth of the Mississippi River, according to the National Hurricane Center's 6 a.m. GMT (2 a.m. ET) advisory.
That's only 15 mph slower than when the storm first crossed land in Cuba. The U.S. National Hurricane Center said the storm could pick up even more strength as it enters the gulf's warmer waters.
The Hurricane Center said Gustav is an "extremely dangerous Category 4 hurricane."
In the United States, state and federal officials -- eager to prove they are ready -- urged residents to flee, and many of those residents obeyed, moving north by the tens of thousands, according to the governor of Louisiana, whose state was hit three years ago by Hurricane Katrina.
New Orleans Mayor Ray Nagin called Gustav "the mother of all storms," saying its destruction could outstrip that from Katrina, which flooded much of his city and killed about 3,000 people.
A hurricane watch remained in effect for the northern Gulf Coast from east of High Island Texas eastward to the Alabama-Florida border, including the city of New Orleans and Lake Pontchartrain, the National Hurricane Center said. A hurricane watch means that hurricane conditions are possible within the watch area, generally within 36 hours.
On its current track, Gustav would reach the northern Gulf on Monday.
Thousands of anxious Cubans had boarded up their homes and sought safety from Gustav earlier as it slammed into the island nation. as it made landfall in western Cuba on Saturday night with sustained winds near 150 mph (240 kmh), the National Hurricane Center said.
Some fluctuations, with an overall slight strengthening, is forecast for the next 24 hours, and forecasters said Gustav could become a Category 5 hurricane within that period. See Gustav's projected path »
Hurricanes are ranked 1 to 5 in intensity on the Saffir-Simpson scale. A Category 5 is the highest classification, with sustained winds of more than 155 mph. A Category 4 has winds of 131 to 155 mph and can cause extreme damage.
Anticipating the storm, about 10,000 Cubans evacuated Saturday to Cuba's mainland from La Isla de Juventud, or Isle of Youth, a province south of the nation's western end. The hurricane center said western Cuba could receive as much as 25 inches of rain.
Residents sought refuge with family or friends, or went to a government shelter, CNN's Morgan Neill said.
Cubans also headed to higher ground on the mainland, with many utilizing horse-drawn carriages and vintage cars.
President Raśl Castro checked with the island's officials Saturday to be sure preparations were on track, Neill said.
Cuba's western province of Pinar del Rio, currently under a hurricane warning, is the center of the nation's lucrative cigar industry.
A hurricane warning remained in effect for the Cuban provinces of Pinar del Rio, la Habana, Ciudad de la Habana, Isla de Juventud, Matanzas and Cienfuegos. A warning indicates that hurricane conditions will arrive in the next 24 hours.
Gustav caused little damage on Grand Cayman on Friday night, according to an official at the Island's airport.
As a tropical storm, Gustav blasted Jamaica with high winds and drenched it with heavy rain Thursday night into Friday. Four people were killed, the National Emergency Operations Center in Jamaica said, and the storm downed trees and damaged houses.
"It is total devastation everywhere," the councilor for the Manchioneal division, Alston Hunter, told the Jamaican Gleaner newspaper. "Several residents are now crammed into disaster shelters here in east Portland, and the weather continues to make the situation worse."
At least 51 people were killed in southwestern Haiti and eight were killed in neighboring Dominican Republic as Gustav roared through as a Category 1 hurricane on Wednesday, officials there said.
The hurricane center's five-day forecast places a landfall anywhere from Galveston, Texas, east to Mobile, Alabama.
New Orleans, Louisiana, is at the center of the projected path.
A hurricane watch remained in effect for the northern Gulf Coast from east of High Island, Texas, to the Alabama-Florida border, including the city of New Orleans and Lake Pontchartrain, the National Hurricane Center said. A watch means hurricane conditions are possible within the specified area, generally within 36 hours.
The Gulf Coast is still recovering from Hurricane Katrina, which killed more than 1,800 people when it slammed ashore August 29, 2005.
Mississippi Gov. Haley Barbour said Hurricane Katrina victims living in government-issued trailers or mobile homes along his state's coast will begin evacuating this weekend.
Gustav is the second major hurricane of the 2008 Atlantic hurricane season.
Meanwhile, Tropical Storm Hanna passed north of the Leeward Islands on Friday with maximum sustained winds of 50 mph (80 kph), the hurricane center said.
The center predicted gradual strengthening, and Hanna could be near hurricane strength Sunday.
Copyright 2008 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.