The Canadian Hurricane Centre warned that Fiona - expected to reach Canada's shores by Saturday morning - could be "a historic, extreme event".
Authorities have warned of potential coastal flooding, power outages and dangerous driving conditions.
At least eight people in Fiona's path through the Caribbean have died.
Canadian forecasters said late on Friday that Fiona - now a category three storm - was packing maximum sustained winds of up to 120 mph (195km/h), though this is projected to decrease as it makes landfall.
"It's going to be a bad one," Prime Minister Justin Trudeau said Friday. "We encourage everyone to stay safe and to listen to the instructions of local authorities and hang in there for the next 24 hours."
Tropical storm warnings have been issued for the Atlantic provinces of Nova Scotia, Prince Edward Island, Newfoundland and New Brunswick, as well as in parts of Quebec.
The country's eastern region could receive up to 10in (25cm) of rain, increasing the risk of flash flooding.
In Nova Scotia, shelters have been prepared in Halifax and Cape Breton - where the storm is projected to make landfall - for people to take cover ahead of the storm.
"We have been through these types of events before, but my fear is, not to this extent," said Amanda McDougall, mayor of Cape Breton Regional Municipality.
"The impacts are going to be large, real and immediate."
Severe hurricanes in Canada are rare, as storms lose their energy once they hit colder waters in the north and become post-tropical instead. But pressure in the region is predicted to be historically low as Hurricane Fiona hits, making way for a heavier storm.
Nova Scotia was last battered by a tropical cyclone in 2003 with Hurricane Juan, a category two storm that killed two people and heavily damaged structures and vegetation.
Meteorologist Bob Robichaud warned on Friday afternoon that Fiona will be bigger than Juan, and stronger than 2019's Hurricane Dorian, which also reached the shores of Nova Scotia.
"It is certainly going to be a historic, extreme event for Atlantic Canada."Heavy rain and winds lashed Bermuda early on Friday, forcing schools and offices to close, before the hurricane headed north.
Fiona had already wreaked havoc on Puerto Rico and the Dominican Republic earlier this week, with many still left without power or running water.
A sick four-month-old infant whose mother was unable to get to the hospital because of blocked roads is among up to four casualties in Puerto Rico. A death was also recorded on the French island of Guadeloupe.
Florida also faces a hurricane threat after a separate tropical cyclone formed in the Caribbean Sea.
Tropical Depression Nine is in its early stages and is moving on a path that could bring it to Florida next week as Hurricane Hermine, according to the US National Hurricane Center.