Police officials said the explosion occurred near the town of Blagoevgrad, 50 miles south of Sofia, and damaged about 30 feet of the pipeline running beneath agricultural land.
The most likely cause of the blast was a gas leak, said Angel Semerdjiev, general manager of Bulgartransgas, an affiliate of state-owned gas company Bulgargaz. "There is no other visible cause for the incident."
Energy Ministry spokeswoman Veselina Velcheva said authorities were investigating the incident. A recent inspection had shown "that everything was in good order," she said.
A police spokesman said safety systems in the pipeline automatically shut off the flow of gas and the situation was under control. The area was cordoned off and measures taken to prevent any risk from the gas to people living nearby.
Repairs were being carried out on the damaged part of the pipeline, and Semerdjiev said the flow of gas should be back to normal within 48 hours. Velcheva said earlier that the work could take up to 78 hours.
Bulgargaz transports gas from Russia's Gazprom through Bulgaria to Greece, Macedonia and Turkey.