The National Hurricane Center said the storm had weakened slightly over Cuba and was likely to become stronger when it enters the Gulf of Mexico. It was expected to hit the Gulf Coast of the United States Monday.

"Everyone's at home. It's getting very ugly," Ofilia Hernandez told MSNBC, speaking from a community telephone in Isla de Juventud, the first area in Cuba hit by the hurricane. "All night last night there was wind, but not like now. Now it's very strong. Things are starting to fall down."

In late afternoon, an Air Force hurricane hunter recorded winds of up to 150 mph, the National Hurricane Center in Miami reported.

"This makes Gustav an extremely dangerous Category 4 hurricane," the center said in an advisory.

The center issued a hurricane watch for the Gulf Coast from High Island in Texas to the Alabama-Florida border and a tropical storm watch for the Florida Panhandle. A watch means hurricane conditions are possible in the area within 36 hours.

At 11 p.m. EDT, the storm was over Cuba with the center about 90 miles west of Havana and 530 miles southeast of the mouth of the Mississippi River. Gustav, with maximum sustained winds of 140 mph, was moving northwest at almost 15 mph. The storm was expected to pass over western Cuba during the night and into the Gulf of Mexico.

In Louisiana, where Hurricane Katrina ravaged New Orleans in 2005, Gov. Bobby Jindal said more than a dozen parishes had declared states of emergency. He said some parishes were beginning mandatory evacuations.

New Orleans declared a mandatory evacuation late Saturday.