The Interior Ministry arrested 10 men it said had collected donations and given them to “suspicious elements”.
Interior and security apparatus sources suggested this involved helping insurgents in Iraq. No formal charges have been laid.
Lawyers and colleagues of the men have said the arrests were designed to scupper plans to set up a political group. Saudi Arabia has no parties or elections to its parliament, and political life is dominated by the ruling Al Saud family.
“These men have spent more than six months in detention pending investigations and we demand their release according to the penal code,” said a statement signed by 76 well-known figures in Saudi society, including liberal, Islamist and Shia activists.
The statement said regulations require prosecutors to extend detention without charge after the expiry of six months, file a formal indictment, or release the men.
The statement came in the form of a letter addressed to the Human Rights Commission which answers directly to King Abdullah.
The men arrested in February are mainly Islamist reformers who have signed reform petitions to the royal family in the past and some have been detained for their activities before.
King Abdullah is viewed by many Saudis as a supporter of some political reforms but diplomats say his room for manoeuvre is hindered by opposition from powerful members of the royal family. His half-brother Prince Nayef is Interior Minister.
Saudi print and television media, which includes many pan-Arab outlets, have not reported the detentions or efforts to release the men. Saudi Arabia is the world’s largest oil exporter and a close US ally.
