Fiona ripped into eastern Canada on Saturday, forcing evacuations, knocking down trees and powerlines, and reducing many homes on the coast to rubble with hurricane-force winds.
"We're seeing reports of significant damage in the region, and recovery is going to be a big effort," Prime Minister Justin Trudeau said, adding that some areas have experienced "storm damage like they've never seen."
He met with the government emergency response team and said the military would be deployed to help with the clean-up.
The Canadian leader was due to attend the funeral of former Japanese Prime Minister Shinzo Abe but said he now would no longer make the trip. Instead, he would visit the storm-damaged areas as soon as possible.
"There are people who see their houses destroyed, people who are very worried, we will be there for you,'' he promised.
Canadians describe devastation as Fiona hits
In Nova Scotia, where the storm made landfall, more than 384,000 households were left without electricity, Nova Scotia Power reported.
New Brunswick reported 32,000 homes were without power, as well as 82,000 in Prince Edward Island.
Nova Scotia Premier Tim Houston said roads were washed out, including his own, and said an "incredible'' amount of trees were down.
"It is pretty devastating. The sad reality is the people who need information are unable to hear it. Their phones are not working, they don't have power or access to the internet," Houston said.
Ocean waves pounded the town of Port Aux Basques on the coast of Newfoundland and washed entire buildings into the sea.
"There is an apartment building that's literally gone. There are entire streets that are gone," Rene Roy, editor-in-chief of Wreckhouse Weekly in Port aux Basques, told the Canadian Broadcasting Corp.
"This is hands down the most terrifying thing I've ever seen in my life," Roy said, describing many homes as "just a pile of rubble in the ocean right now."
Darlene Compton, deputy premier of Prince Edward Island, said it had been a "nerve wracking" night. "This morning we all woke up to some very scary scenes, roads washed down, uprooted trees, mail boxes where they are not supposed to be."
Fiona leaves trail of destruction
Fiona had already wreaked havoc on Puerto Rico and the Dominican Republic earlier this week.
Meanwhile, in the Caribbean, Tropical Storm Ian was predicted to rapidly strengthen and hit Cuba early Tuesday as a hurricane and then hit southern Florida on Wednesday or Thursday, the US National Hurricane Center said.