Western Saharan campaigners says Morocco should not rejoin the African Union unless it withdraws from the disputed territory.
Morocco applied to rejoin the AU in July, after pulling out in 1984, when the body recognised Western Sahara's independence.
King Mohammed VI of Morocco has toured various African countries in recent weeks seeking support for the bid.
Moroccan troops went into Western Sahara after Spain withdrew in 1975.
It says Western Sahara is part of its historic territory.
Mohamed Salek Ould Salek, Foreign Minister of the Polisario Front, recognised as Western Sahara's government by the AU, told the BBC that Morocco's occupation contradicted the founding principles of the AU.
"The aim and essence of the Union is the liberation of Africa. Morocco [has] become a coloniser," he said.
"We fought against colonialism and apartheid; how come we are going to accept a country who declared publicly and solemnly that he is occupying a part of a country who is a member of the African Union?" Mr Salek asked.
He insists that if Morocco "wants to join the club, [it] must respect the rules of the club".
Morocco's application will be discussed at the AU summit in January.
How did we get here?
- 1975-76: Morocco annexes two-thirds of Western Sahara after colonial power Spain withdraws.
- 1975-76: Polisario Front declares the Saharan Arab Democratic Republic (SADR), with a government-in-exile in Algeria. Thousands of Sahrawi refugees flee to western Algeria to set up camps.
- 1984: Morocco leaves the Organisation of African Unity (which later became the African Union) in protest at the SADR's admission to the body.
- 1991: UN-monitored ceasefire begins in Western Sahara, but the territory's status remains undecided and ceasefire violations are reported. The following decade sees much wrangling over a proposed referendum on the future of the territory but the deadlock is not broken.
- March 2016: Morocco threatens to pull its soldiers out of UN global peacekeeping missions in Western Sahara, after UN Secretary-General Ban Ki-moon uses the term "occupation" when referring to the territory.
- May 2016: Long-time Polisario Front leader Mohamed Abdelaziz dies aged 68