Moroccan King Mohammed VI is making his first official visit to the Western Sahara since 2006 to mark the 40th anniversary of the country's annexation of the mineral rich territory.

In addition to commemorating the 1975 annexation Morocco calls the "Green March," the visit is part of a campaign to promote Morocco's decentralization plan and boost investment.

The visit, which began Friday, has sparked anger among Sahrawis in the territory, and in refugee camps outside Tindouf, Algeria, run by the Polisario Front pro-independence movement.

Rights groups say Morocco uses violence to stifle dissent. On Thursday, Human Right Watch called for the release or retrial of 21 Sahrawis arrested for their role in the 2010 Gdeim Izik protest camp outside Laayoune.