Polisario movement members are meeting this Friday in a refugee camp in Algeria to discuss leadership elections.
Morocco controls almost 80% of the resource-rich territory and has proposed an autonomy plan under its sovereignty.
The Sahrawi Polisario Front, backed by Algeria, has called for a referendum on self-determination.
More than 2,200 members of the movement and 370 foreign guests are expected to attend the five-day congress.
The rift has already led to the diplomatic breakdown of relations between Algeria and Morocco.
The Polisario’s current head Brahim Ghali, 73, appears to enjoy the vital backing of Algeria and is expected to be re-elected at the gathering
The conflict in Western Sahara dates back to 1975, when colonial power Spain withdrew from the region, sparking a bitter 15-year war between the Polisario and Morocco for control of the territory.