It thought about 50 coaches were stored at the depot at an industrial estate in Coatbridge and around half were reduced to burnt out shells.
A row of charred metal was all that remained of one bus fleet. Early estimates suggest the damage could be about £3million.
Emergency services were called to Greenhill Industrial Estate about 2am after reports several buses were on fire. Residents said they could see flames and hear explosions from the site, which is used by several firms, including A Trip In Time and Arthur's Coaches.
Around 40 firefighters tackled the blaze and tried to cool cylinders inside the yard.
The depot is near a timber yard and homes and families were evacuated in case the flames spread.
Residents in Cromlet Avenue, which backs on to the site, were woken and told they would have to leave.
Aroun 60 homes and 200 residents had to flee. Some were taken to the nearby Greenhills Primary, but most went to friends and relatives.
One man, who was evacuated with his wife and three children, said the fire was well alight by the time they left.
He said: "We could hear explosions and thought it was time to leave. We feared the fire would spread and didn't want to take any risks.
"Firefighters were knocking on doors telling people to leave."
Firefighters were still at the scene this morning monitoring hotspots and damping down areas. They also gained access to acetylene cylinders and started cooling them.
George McGrandles, area commander for Lanarkshire for Strathclyde Fire and Rescue Service, later said on the radio: "Fifty buses was quite an intense fire."
Drivers turned up for work this morning and were unsure of what would happen.
One driver said: "We have been told to go to another site, but we don't know any more than that."