The Moroccan authorities have prevented a second delegation of Spanish MPs from visiting Spain's former colony of Western Sahara.
The Spanish delegation was forced to return to the Canary Islands
The politicians, who had planned to investigate human rights in the disputed territory, were prevented from leaving the plane at Laayoune airport.
There has been no official explanation. A Moroccan news report said the MPs supported a pro-independence group.
A group of Spanish MPs and journalists was also refused entry on Sunday.
"It is surprising that four deputies who represent four parties who simply wanted to hear what was happening in the Sahara have not been allowed to enter," Joan Herrera, a left-wing national MP, told Spain's Cadena Ser radio.
Protesters gathered outside Madrid's Moroccan embassy
The mineral-rich territory of Western Sahara was seized by Morocco after Spain withdrew in 1975.
The official Moroccan news agency (MAP) accused the mostly Catalan delegation of sympathising with the Polisario Front rebels, who fought Morocco after annexation and are now based in camps in southern Algeria.
Last month, Algeria reiterated its support for Polisario.
That led to the postponement of a summit of North African heads of state, when King Mohammed VI of Morocco said he would not attend because of the dispute with Algeria over Western Sahara.
The United Nations views Western Sahara as a disputed territory.
Moroccan-Spanish relations have thawed since a Socialist government was elected in Spain in March 2004, replacing the conservative administration of the Popular Party.
Since the Casablanca and Madrid bombings, the two countries have stepped up their anti-terror co-operation.
In January, King Juan Carlos of Spain called for a fair, final settlement of the Western Sahara dispute, in a speech to the Moroccan parliament in Rabat.
He said Spain wanted to see a "consensual, just and final" settlement for the former Spanish colony.