The official Moroccan news agency said on Monday that the group was barred from Western Sahara's capital, Laayoune, due to its separatist support - a reference to the Polisario Front that has sought independence for decades.
Members of the group were well-known for their "biased, subjective and unconditional support for Polisario", the agency said.
The delegation, which included members of non-governmental organisations, was on a fact-finding mission concerning the human rights situation in the former Spanish colony after last month's riots in Laayoune.
The delegation flew to the city from Las Palmas in the Canary Islands but was not allowed to get off when the plane landed, according to a Spanish Foreign Ministry spokesman and members of the group.
Spain's Foreign Ministry asked Moroccan authorities to allow the group to disembark but Morocco made them fly back to Las Palmas, according to Madrid city council member Ines Sabanes.
"When we arrived at the airport of Laayoune, they told us over the loudspeaker that we couldn't move from the plane," Sabanes told state radio.
In a related development last Friday, Moroccan security forces expelled Spanish journalist Maria Cristina Berasain.
The government news agency said she had first claimed to be a pharmacist on holidays but later confirmed that she worked for the Berria website and that she came to Morocco to cover the Laayoune riots.
On 29 May, 33 Sahrawi men were charged with taking part in the riots in the vast territory Morocco seized after colonial power Spain withdrew in 1975.
Spanish media gave extensive coverage to riots. The daily newspaper El Pais said Moroccan flags were burned and Molotov cocktails thrown at riot police.
Morocco described the events as an uprising by Polisario, which threatened last month to resume its armed struggle against Morocco if there was no breakthrough in UN-led peace talks in the next six months.
Polisario conducted a low-intensity guerrilla war until the United Nations brokered a ceasefire in 1991, with the promise of a referendum to decide the territory's fate.
Disagreements over who is eligible to vote have prevented the referendum from taking place.