The blaze has destroyed a number of homes, dropping ash on the streets of the city in the province of Alberta.
Fleeing residents have caused gridlock on the main road leading from the city, 380km (235 miles) north of Edmonton.
The evacuation from Fort McMurray - which lies in an oil sands region - is the biggest in Alberta's history.
"If you just walk outside, you feel it (ash) falling on you. You see it floating in the air. I can take a broom and brush it off my deck,"  resident Mark Durocher was quoted as saying by the Globe and Mail.
Homes in at least two neighbourhood have been gutted, and the fire has now spread to Highway 63 - the main road into Fort McMurray from the south.
Firefighters are continuing to tackle the blaze, but the local authorities have called for reinforcements, including a water-dumping helicopter.
So far there have been no reports of any injuries.

The Lady Helen had been sailing from Martin's Haven to nearby Skomer Island.
A mayday was sent out and passengers were transferred to rescue vessels after the boat started taking in water as she was towed back to the mainland.
Milford Haven coastguard said all 48 passengers were "safely ashore with no injuries".
Boat trips are popular to Skomer, a wildlife sanctuary and home to colonies of thousands of sea birds just off the Pembrokeshire coast.
Lady Helen was heading from Martin's Haven to Skomer when she ran aground on a rock at Little Sound, said the coastguard.
A mayday alert was issued at 12:55 BST and the RNLI lifeboats from Angle and St David's were scrambled, along with a Dyfed-Powys Police rib and an RAF helicopter from Chivenor in Devon.
Sister boat the Dale Princess managed to drag the Lady Helen off the rock using a line but she began taking in water as they headed back to shore.
Milford Haven Coastguard watch manager Barrie Yelland told BBC Wales: "Several vessels in the area also responded and went to the assistance of Lady Helen.
"They started to tow her back to Martin's Haven but unfortunately she started to sink and they decided to transfer all the passengers to the other vessels.
"It's something we try not to do unless we have to because there's a danger in transferring people at sea but it came apparent there was no option."
All 48 passengers were reported to be safely ashore with no injuries.
"Everyone has done an excellent job carrying out this rescue, including all the various vessels that responded," said Mr Yelland.
"Special thanks must also go out to the boat Over Dale," said the watch manager.
One of those helping passengers was diver Dr Pauline Crossland, on board Over Dale.
"We were a group of divers who were on our way out and a mayday call was put out," she said.
"A number of boats had to go to its rescue.
"We had quite a few children and elderly ladies on our boat. The children were initially a bit scared. They said the boat had been listing. But once we gave them a Twix each they were fine."
Lady Helen was beached at Martin's Haven and there are three salvage pumps on board and she was being pulled further up the shore.
A huge wildfire has forced the evacuation of about 60,000 people from Fort McMurray - the entire population of the Canadian city.
[NEXT_CONCEPT]
A total of 48 passengers, including children, have been rescued from a boat after it struck a rock and was in danger of sinking off Pembrokeshire.