Crews were alerted just before 3pm on Easter Monday to the blaze near Aberlour, Moray.
Dry conditions and high winds caused the flames to spread aggressively, affecting an estimated 50 square kilometres of gorse, grass and forestry, the Scottish Fire and Rescue Service (SFRS) said.
At its height, around 80 firefighters, two helicopters, 19 fire engines and specialist resources were called upon to help tackle the fire, which was on four fronts.
Firefighters are still at the scene and they are expected to remain there for several days.
Area manager Bruce Farquharson was the incident commander and is now overseeing it from the SFRS operations control centre.
He said: “This is shaping up to be one of the largest wildfires that the UK has seen in years.
“The conditions, including the weather, the terrain and the sheer scale of the incident have made it very challenging.
“This type of incident requires a large amount of resources, and we have called upon the resilience of our national service to tackle it.
“Additionally, we have received additional support from our partners - which includes two helicopters which have been working alongside our crews to tackle the fire in difficult-to-reach areas.”
Several properties close to the fire have been evacuated as a precaution and the blaze created a large smoke plume which could be seen from space, the fire service said.
Mr Farquharson said: “We are advising people who live in the path of the smoke to keep their windows and doors closed as a precautionary measure.
“Additionally, many roads in the area remain closed and we are asking people to avoid the area for their own safety, and to allow full access for the emergency services.”
He added: “We will remain on scene for several days yet.
“Our crews continue to work hard and in challenging conditions to contain and extinguish the fire, and they must be commended for their efforts.
“Additionally, the continued support of our partners and emergency service colleagues has been invaluable and our sincere thanks goes to them also.”