Dozens of Saudis protested outside the Interior Ministry in Riyadh Sunday to demand the release of prisoners.
Large numbers of riot police were guarding the ministry, and witnesses say a number of protesters were arrested after trying to go into the building.
Protests are banned in Saudi Arabia, and King Abdullah has been offering incentives to prevent the unrest sweeping the Arab world from taking hold in his country. Several small demonstrations have taken place.
On Friday, the king promised reforms and cash incentives to residents.
The 86-year-old monarch returned to Saudi Arabia in February after spending three months abroad for medical treatment.
Some information for this report was provided by AP and Reuters.