HAVANA, Nov 6 (Reuters) - Cuba's national electrical grid collapsed on Wednesday as Hurricane Rafael slammed into the island's southwest shore, packing sustained winds of 115 mph (185 kph) and wreaking havoc on the already crisis-stricken country.

The hurricane was churning about 45 miles (75 km) southwest of Cuba's capital Havana late in the afternoon, lashing the capital city of nearly two million people with driving rain and violent wind gusts.

The Miami-based NHC warned of a "life-threatening storm surge, damaging hurricane-force winds and flash flooding" across much of western Cuba. The region, including Havana, remained under a hurricane warning.

Cuba's state-run grid operator UNE said the high winds had caused the country's electrical system to collapse. State-run television reported the entire population of 10 million people was without electricity - the second such incident in less than a month on the island.

Conditions had deteriorated quickly by mid-afternoon in Havana, east of the storm's predicted track, and wind and rain had already downed trees and powerlines on city streets.

The capital of 2 million residents is especially vulnerable to a hurricane strike and flooding, with antiquated, densely packed housing and decrepit infrastructure.

Police cruisers with loudspeakers began circling central neighborhoods encouraging people to shelter in place ahead of the storm.

Schools and public transportation in the city were suspended until further notice, and authorities grounded flights at both Havana's Jose Marti International Airport as well as at the popular beach resort at Varadero through Thursday.

Officials said they had evacuated more than 100 Canadian tourists from Cayo Largo, another popular beach destination off southwestern Cuba.

The farm provinces of Artemisa and Pinar del Rio - home to the prized tobacco used in Cuba's famous hand-rolled cigars - were expected to take a near direct hit as Rafael made landfall on the Caribbean island. Farmers had moved to protect 8,000 metric tonnes of tobacco in the area, Agriculture Minister Ydael Pérez Brito said, as well as ripening fruits and vegetables.

The timing could not be worse for the Communist-run island, which just last month suffered another total collapse of its national electric grid, leaving the country without power for several days.

Hurricane Oscar made landfall in far eastern Cuba around the same time as the blackout, throwing a one-two punch that has sapped precious resources in a country suffering from severe shortages of food, fuel and medicine.

Cuba's oil-fired power plants, already obsolete and struggling to keep the lights on, reached a full crisis this year as oil imports from Venezuela, Russia and Mexico dwindled.

The hurricane is the latest blow to the country's already precarious grid and infrastructure.

Rafael grazed the Cayman Islands as a Category 1 cyclone overnight before increasing in less than 24 hours to a much more powerful Category 3 storm off Cuba's southwestern shore.

Forecasters predict Rafael will spin off towards the western Gulf of Mexico later this week, though the track remains uncertain, the NHC said.

Sign up here.

Reporting by Dave Sherwood in Havana, additional reporting by Nelson Acosta, Editing by Christina Fincher, Marguerita Choy and Sandra Maler