Dozens of protesters in Riyadh on Tuesday called for the release or immediate trial of imprisoned Islamist relatives, witnesses said.
Around 40 people, including five women, demonstrated outside Saudi Arabia's Human Rights Commission, before being dispersed by police. No violence was reported.
Riyadh warned in October it would deal "firmly" with demonstrations, which are banned in the kingdom, after several demonstrators were arrested in September outside a prison north of the capital.
Human rights watchdog Amnesty International slammed the warning and urged the authorities to "withdraw their threat."
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The protesters' banners read "release innocent detainees" and "release Heila al-Qsayer," the first woman to be arrested in the kingdom for involvement in Al-Qaeda-linked violence.
Qsayer was sentenced last year to 15 years in jail, to be followed by a 15-year travel ban.
Tuesday's protesters were relatives of "radical Islamist" prisoners, some of whom are held without charge, human rights activists said.
A wave of deadly Al-Qaeda attacks in the kingdom between 2003 and 2006 prompted the authorities to launch a crackdown on the local branch of slain Saudi-born Osama bin Laden's group.
An independent Saudi rights organisation says there are some 30,000 political prisoners in the Gulf kingdom, a charge Riyadh denies saying there are none.