The bus, which was not carrying any passengers, failed to stop as it drove down a hill because of a technical fault, the Interior Ministry said in a statement.
"The tragedy ... took the lives of 16 people," it said.
A 16-year-old boy was among the dead, and police said 15 people were admitted to hospital in the city of Yambol, some 260 km (162 miles) southeast of Sofia.
The bus driver suffered minor injuries. Earlier reports said two passengers were among the fatalities.
The accident happened near the site of a local fair crowded with people who were celebrating Orthodox Ascension Day.
Prime Minister Sergei Stanishev interrupted his election campaign tour ahead of a July 5th parliamentary vote and headed to the site of the accident, the government said in a statement. President Georgi Parvanov also headed to Yambol.
The cabinet called a national day of mourning tomorrow.
Reuters