Tibetan Empire had nine high ministers in court. The highest one was "gung lön chen po" (གུང་བློན་ཆེན་པོ, "lön chen" (བློན་ཆེན) for short), which could be translated into English as "Chief Minister" or "Prime minister".
The first "Lönchen" was appointed during Detrul Namshungtsen's reign. It was abolished before the Era of Fragmentation.
Here is a complete list of "Lönchen" according to "The Old Tibetan Chronicle" (P.T. 1287).