Police said the bus was taking tourists on a weekend trip to a resort when the bus overturned at 5.10pm local time yesterday.
The bus was reported to have collided with three oncoming cars before veering off the road, the Sofia Globe reported.
The major of Bozhurishte, a town that is around a 30-minute drive north of Sofia, told reporters that all of the passengers were from his town.
At least 15 ambulances from Sofia, Montana and Vratsa took the injured to hospitals in the capital and doctors said some are in a critical condition.
The health minister Kiril Ananiev confirmed the tragedy yesterday and made the initial death toll 15 but doctors at Sofia’s emergency hospital said another victim died that night.
Interior minister Valentin Radev said that 34 people, including the driver, had been travelling on the bus.
The driver was taken to Tsaritsa Joanna Hospital in Sofia.
Prime minister Boiko Borissov has called for a national day of mourning for the victims tomorrow.
The news follows another tragic bus crash near Sofia in April this year that killed six people, including a child, and left 22 injured.
Bulgarian authorities said that a passenger bus had overturned on the motorway 25 kilometres east of Sofia on April 13.
The accident occurred after a car crashed into the bus causing the driver to lose control of the vehicle and flip into a roadside ditch.
The driver was taken into custody for questioning over the incident.
The bus was taking passengers from Burgas, a town on the Black Sea coast, to Sofia, a three and a half hours drive.