"The number of people arbitrarily detained in Saudi prisons today is estimated to be thousands," said senior legal consultant, Deborah Manning of Alkarama, a Geneva-based organisation focusing on human rights violations in the Arab world. "They are being held in clear violation of international human rights law and the principles of natural justice. In addition to these abuses of process those imprisoned are also frequently subjected to torture and experience other oppressive physical and psychological hardships. .

"We argue that anyone arrested in Saudi Arabia deserves to be charged, tried and given access to a lawyer - or returned to freedom.

The Adala association (www.adalaksa.org), an organisation that provides support for the families of Saudi's arbitrary detainees, used the event to launch its campaign.

Speaking at the gathering outside the Saudi embassy in London, the wife of Abdelhakim Gellani, held in Riyadh prison, said, "I understand if someone does something, you charge them. But my husband has been detained three years and has never been charged. Adala means justice, and this is all we are asking for."

Saudi Arabia's human rights record has come under attack in recent years for the government's use of arbitrary detention to gag opposition groups promoting for reforms in the Kingdom.

Freedom of association and the basic rights of freedom of expression are banned in Saudi Arabia, meaning that opposition groups, human rights activists and writers critical of the regime are seen as instigators of "disunity, sedition and separation" rather than defenders of universal human rights.

"Many of the basic rights of freedom of speech and association in Saudi Arabia are curtailed by vague definitions of criminality and the government's counter-terrorism policies," said Manning.

Article 39 of the Basic Law of the Kingdom of Saudi Arabia, enacted in January 1992, states that "all acts that foster ... division or harm the state's ... public relations shall be prohibited."