On Saturday, more than one million people from the Florida Panhandle to Louisiana were under evacuation orders. Landfall was expected on Sunday afternoon anywhere from the Florida Panhandle to southeast Louisiana.
"This is a very dangerous storm, and we hope that you will evacuate," Florida Governor Jeb Bush said to residents in the Panhandle.
The storm, the earliest to reach Category 4 strength in the Caribbean on record, was expected to bring up to 20cm of rain and 1.8m storm surges on Saturday.
In Haiti, at least 100 people were missing in the southwestern peninsula after floods, mudslides and the collapse of a bridge triggered by Dennis, a UN official said on Saturday. There were 10 confirmed deaths.
UN peacekeepers and local rescue workers had been searching for 40 people reported missing after an overflowing river tore down a bridge in the southwestern town of Grand Goave, said Commander Mark Breaud, the leader of the UN civilian police for the region.
The bodies of five people killed in the incident were recovered.
Families and residents reported at least another 60 people missing from flooding and mudslides, many of them in remote, mountainous communities that UN troops and rescuers have been unable to reach, Breaud said.
Dennis sideswiped dangerously deforested Haiti on Thursday.
Civil Protection officials said at least four people were killed in the southern town of Jacmel but could not immediately provide details. The 10th death occurred when an uprooted palm tree crushed a hut in the southwestern town of Les Cayes.
Dennis also was blamed for at least 10 deaths in Cuba.
Gusts from Hurricane Dennis
batter the Havana seawall
Several tornado touchdowns in the Tampa Bay area caused minor damage such as downed trees, and more tornadoes and battering waves were likely in parts of the Gulf of Mexico coast on Sunday.
The storm decreased in strength to Category 1 after passing over Cuba, but strengthened again as it moved over open water into a Category 2, with top winds of 161kph.
More than 211,000 homes and businesses were without power on Saturday in the southern tip of Florida, including the city of Key West, officials said.
Eye passes Key West
The hurricane's eye passed west of the island on Saturday morning, but it still produced stinging rain and wind gusts that buckled windows. Tree branches, plywood, street signs and other debris littered the streets, and awnings hung precariously from storefronts.
Waves crashed over a seawall, sending sand and coral onto a main road. About three blocks of the tourist drag of Duval Street were under 30cm of water.
No injuries were reported.
Traffic doubled on some Mississippi roads as people fled Florida, Alabama and Louisiana. Alabama officials were turning Interstate 65 into a one-way route north from the coast to Montgomery.
"All day long, all of our phones have been ringing. The only thing we can tell people is that we are sold out," said Lasonya Lewis, a clerk at a Montgomery hotel.
About a half-million people in coastal Alabama and more than 700,000 in the Keys and low-lying areas of the Florida Gulf Coast were under evacuation orders.
Key West felt sustained winds around 98kph early on Saturday, with gusts up to 119kph, said meteorologist Matt Strahan.
At 1300 pm EDT (1700 GMT), hurricane-force wind of at least 119kph extended up to 56km from Dennis's centre, and tropical storm-force wind stretched up to 281km out.
Many in Dennis's strike zones were aware that it was following nearly in the path of Ivan, which came ashore at the Florida-Alabama line, causing 29 deaths and $4 billion damage in the Panhandle.