Human rights workers say they have faced increasing threats and pressure for doing their work over the past two years from police.
Both sides are at loggerheads over a small stretch of land west of the Preah Vihear temple.
Kem Sokha is on a trip through the US to build support for his party ahead of commune council elections in 2012.
Villagers at the Kampong Speu concession say they are being pushed off 2,000 hectares of land as the company expands.
Members include senior government adviser Om Yientang, who is the head of the unit, and his three deputies.
The convention was only the second for the party since its formation in 2007.