Bulgarian customs officials confiscated more than 288kg of heroin hidden on a truck from Iran, prosecutors announced on Thursday.
The haul is the biggest amount of heroin seized at Bulgaria's borders this year, the customs agency said.
Two men - the Iranian truck driver and a Turkish man, who was allegedly to receive the drugs in Bulgaria - were detained and indicted for drug trafficking, the Haskovo regional prosecution said in a statement.
They risk jail terms from 15 to 20 years, it added.
The drugs were placed in 144 packages hidden inside the floor and ceiling of a spray painting machine transported inside the truck.
It was found when the vehicle was x-rayed upon entering Bulgaria from Turkey at the southeastern Kapitan Andreevo border checkpoint on Sunday, but the seizure was only announced on Thursday.
Bulgaria, which lies on the so-called Balkan drugs route from the Middle East to Western Europe, has seen a several-fold increase in heroin seizures over the past three years.
In 2018, the customs agency confiscated a total of 994 kilos of heroin at the country's borders, a rise of 13% from 2017.
The amount included the biggest heroin haul in the agency's whole history of over 712kg of heroin found in two Iranian trucks travelling to Austria in October last year.
Over the past week Bulgarian authorities also found over 210kg of cocaine washed ashore or floating in life jackets in the Black Sea, just days after similar packages were seized along the coast in neighbouring Romania.