Tropical storm Isaac has started started lashing the Florida Keys amid fears it could strengthen into a dangerous hurricane.
Residents and businesses boarded up windows, laid down sand bags as people piled into shelters, while scores of tourists were left stranded after the island's two airports closed.
There were scattered power cuts from Key West to Fort Lauderdale on the Atlantic Coast affecting more than 6,000 people, and flooding in some areas.
At Miami International Airport, more than 450 flights were cancelled on Sunday.
"It's a little scary," said Michelle Remillard, who was trying to get a seat on a flight to New Orleans.
"But I need to get home, you know? And if the storm comes my way again, who knows, I might have to come back here."
Isaac is expected to hit the Gulf Coast next and strike somewhere between the New Orleans area and northwestern Florida on Wednesday - the seventh anniversary of Hurricane Katrina.
It hit the Florida coastline after sweeping across Cuba and ravaging the shores of Haiti, damaging hundreds of homes and killing at least seven people in a country still struggling in the wake of a massive earthquake two years ago.
The deaths reported in Haiti included a 10-year-old girl who had a wall fall on her, as wind and rain lashed a nation trying to recover from the 2010 earthquake.
The government also reported "considerable damage" to agriculture and homes, nearly 8,000 people who had to be evacuated from their houses or quake shelters and more than 4,000 who had to be taken to temporary shelters.
Isaac's centre then made landfall near the far-eastern tip of Cuba, downing trees and power lines. In the picturesque city of Baracoa, the storm surge flooded the seaside Malecon and a block inland, destroying homes.
Forecasters said Isaac was near hurricane strength when the eye of the storm landed in Haiti, where hundreds of thousands of people are still living in squalid, makeshift camps with nothing but sheets of metal or tarpaulin as roofs.
About 400,000 people still live in temporary tent camps following the earthquake that killed 250,000 and seriously damaged the capital city Port-au-Prince in 2010, and they have nowhere to go.
More than 50 tents in one quake settlement collapsed, forcing people to scramble through the mud in an attempt to save their belongings.
In the shantytown of Cite Soleil, just north of the capital, some 300 homes were ravaged by the storm and were either flooded or had their roofs blown off.
"From last night, we're in misery," said resident Jean-Gymar Joseph.
"All our children are sleeping in the mud, in the rain."
Isaac was just west of Port-au-Prince as it swirled across Haiti at 20km (13 miles) per hour, the American National Hurricane Centre said.
The hurricane threat has forced the US Republican Party to delay its national convention by one-day.
Tens of thousands of Republicans will be in Tampa, Florida, for speeches, parties and the formal nomination of former Massachusetts governor Mitt Romney as the candidate to take on President Barack Obama in November 6 elections.
Party chairman Reince Priebus said in a statement that the convention would convene on Monday as scheduled but then immediately recess until Tuesday afternoon.
Florida Governor Rick Stott has declared a state of emergency and residents have been urged to prepare for the worst.
Holidaymakers in Florida Keys off the southern tip of the United States have been urged to leave the area.