The men, most of them academics, have been identified as Matruk al-Falih, Abd Allah al-Hamad, Tawfiq al-Qasir, Muhammad Said al-Taib and Khalid al-Hamid.
They were among 116 signatories of a petition to the government in December calling for transforming the kingdom into a constitutional monarchy.
Falih, a professor of political science at King Fahd University in Riyadh, told AFP by telephone that Hamad, Qasir and himself were brought in the morning to the headquarters of the intelligence services for questioning.
He was unable to provide further details, but a member of his family told AFP later that "they have not heard from Falih since this morning when he was arrested at the university."
Meanwhile the director of the London-based Saudi Human Rights Cenrer, Abd al-Aziz Khamis, told AFP by telephone al-Hamid and al-Taib were "arrested on Tuesday" at King Fahd University and the airport in the western city of Jeddah, respectively.
Khamis said the arrests "might be due to plans by these reformists to create an independent human rights organisation."
Last week Saudi Arabia approved the National Human Rights Association, the kingdom's first human rights watchdog. It has also announced plans to set up a government rights body as part of limited steps towards reform it says must be consistent with Islamic tenets and local customs and not tailored to Western specifications.
The kingdom is often assailed by Western human rights groups for alleged abuses.