By Anthony Boadle
HAVANA, July 9 (Reuters) - Deadly Hurricane Dennis pummeled Havana into the early morning hours on Saturday, uprooting trees and leaving the city's 2.2 million people in darkness after earlier killing 32 people in southeastern Cuba and Haiti.
The storm weakened as it crossed Cuba from a ferocious Category 4 hurricane on the five-step Saffir-Simpson scale to a less severe Category 2, but its 110-mph (180 kph) winds still screamed through the deserted streets of the Cuban capital, where many live in decrepit colonial buildings.
Residents said trees were plucked from the ground.
The U.S. National Hurricane Center in Miami said Dennis -- the strongest hurricane recorded so early in an Atlantic storm season -- would strengthen again on Saturday as it re-emerged over warm open water and skipped south of the Florida Keys.
Thousands of tourists and residents were evacuated from the vulnerable and low-lying island chain at the southern tip of the Florida peninsula, and many more prepared to leave coastal homes along the U.S. Gulf Coast, where Dennis was expected to land on Sunday evening.
Natural gas and oil production was also curtailed as oil platforms in the U.S. Gulf, where a quarter of U.S. crude and natural gas comes from, were evacuated.
Cuban authorities had evacuated more than 600,000 people in different parts of the country as Dennis approached the southern city of Cienfuegos. But the measures, which usually allow the Communist island to escape hurricane strikes with minimal casualties, failed to prevent 10 deaths on Thursday night.
Cuban President Fidel Castro said most of the victims died in collapsed houses in two coastal towns in Granma province. An 18-day-old baby was among those who died, he said on state television, calling the hurricane a "diabolical force."
Officials said 15,400 of the adjacent towns' 20,000 homes were destroyed or damaged. Television images showed rows of clapboard houses flattened by the storm.
On Friday, gusts of up to 149 mph (240 kph) also caused extensive damage in Cienfuegos, where the storm made landfall. It ripped up trees and downed electricity lines, but no casualties were reported there.
In southern Haiti, 15 people died when a swollen river tore away a bridge. The total number of deaths in Haiti reached 22, according to various officials.
At 12 a.m. (0400 GMT), Dennis was located 15 miles (25 km) east-northeast of Havana and about 105 miles (170 km) south-southwest of Key West, where gale-force winds could be felt. But the Florida Keys appeared likely to escape the full brunt of the storm as it moved northwestward at 14 mph (23 kph).
In the U.S. Gulf, energy companies said they were pulling workers off oil rigs and shutting down some crude and natural gas production.
Dennis was on a similar trajectory as last September's Hurricane Ivan, which caused extensive damage to pipelines and rigs, and the approach of Dennis has helped keep U.S. crude futures prices high.
Ivan was one of four hurricanes that struck Florida in a six-week period last year, leaving the state jittery even before the current Atlantic hurricane season began.
Alabama Gov. Bob Riley on Friday ordered the mandatory evacuation of nearly half a million coastal residents in the projected path of Dennis, which the National Weather Service said was likely to hit near the Florida-Alabama border.
NASA decided to leave space shuttle Discovery on its launch pad at Cape Canaveral, Florida. A decision to roll Discovery back to its hangar because of the storm would have delayed a launch scheduled for Wednesday -- the first shuttle mission since the Columbia disaster in 2003.
(Additional reporting by Joseph Guyler Delva in Port-au-Prince, Michael Christie and Jim Loney in Miami, Michael Peltier in Tallahassee, Cathy Donelson in Mobile and Irene Klotz at Cape Canaveral)