Outnumbered by anti-riot police, dozens of Saudi men and women protested outside Riyadh's Interior Ministry, demanding the release of thousands of detainees held without trial for years.
Witnesses say a number of protesters were arrested today after trying to push their way into the building, heavily fortified by about 2,000 special forces and some 200 police vehicles.
There was no official confirmation on the arrests.
It was the third protest this month by families and activists demanding information on the fate of people held without trial for years on security and terrorism charges.
Saudi authorities ban demonstrations and appear increasingly determined to prevent the spread of unrest inspired by uprisings across the Arab world.