Hurricane Dorian made landfall in Canada last night south of Halifax with ferocious 150kph (150km) winds and torrential rains, meteorologists said, toppling trees and whipping up debris.
The storm churned up 20m (65ft) waves which pounded the coast near the port city that is home to Canada's Atlantic fleet.
As it moved north from the US after devastating the Bahamas, the storm was now being called a "very intense post-tropical cyclone," but the Canadian Hurricane Centre warned that it was still packing winds equivalent to those of a Category 2 hurricane.
"We're talking about a very dangerous storm," Bob Robichaud of the Canadian Hurricane Centre told a briefing.
Officials also said it had already dropped more than 100mm (four inches) of rain on Nova Scotia, which could double by this morning.
Storm surges were causing widespread flooding. And more than 450,000 households were without electricity after winds knocked down power lines.
The military was mobilised to deliver aid and help with evacuations, while roads and bridges in the region were closed.
Hurricane Dorian, the most powerful hurricane on record to hit the Bahamas, parked itself over part of the archipelago for almost two days earlier this week, pummelling it with Category 5 winds, with some gusts topping 320kph.
Bahamas leaders believe hundreds and perhaps thousands are missing in the nation of about 400,000 people.
The official death toll, which stood at 43 on Friday, was expected to rises sharply.