"For now, the number of the dead is 15. Four others were severely injured and seven people suffered minor injuries," said a spokeswoman for the regional police department in the city of Yambol, some 260 km (162 miles) south-east of Sofia.

Two of the bus passengers, elderly people from a local care home, were among the fatalities.

According to civil defence officials in Yambol, the total number of injured was 24, state news agency BTA reported.

The Interior Ministry in Sofia said the bus had failed to pull over due to a technical problem. 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 5 parliamentary vote and headed to the site of the accident, the government said in a statement.

The cabinet will call a national day of mourning, it said.