Saudi police arrested 176 people in the Gulf kingdom's central Qassim province on Friday after a protest calling for fair treatment for security prisoners.
State news agency SPA said those detained, who included 15 women, had refused to disperse from a sit-in staged outside the investigation and prosecution bureau in the town of Buraida.
It was the latest in a string of small-scale demonstrations in Qassim and the capital Riyadh in the past two years demanding better treatment of prisoners held on security grounds.
Rights groups say thousands have been detained in the name of security in Saudi Arabia, many of them imprisoned without a fair hearing or held for long periods without trial. They say some were detained merely for demanding political change.
The authorities deny holding political prisoners and say all the security detainees are suspected Islamist militants. They have said more than 5,000 people were detained last year in a crackdown on the militants and most had already been tried.
SPA quoted a police spokesman in the Qassim region as saying those detained on Friday "refused to respond to instructions and attempts by security personnel for more than 12 hours to get them to end their informal gathering".
The sit-in was "an attempt to rouse public opinion by exploiting the cases of a number of persons convicted or accused of crimes or activities of a deviant group," the spokesman said.
Conservative Sunni Muslim clerics hold powerful positions in Saudi Arabia, the birthplace of Islam. However, some have also opposed the ruling family on issues ranging from social reform to the campaign against Islamist radicals.
All protests in Saudi Arabia, the world's top oil exporter, are illegal. The government says it does not mistreat prisoners.
In September, then Interior Minister Prince Ahmed said no further protests about detainees would be tolerated.
The kingdom cracked down on Islamist militants after a series of al Qaeda attacks on government and Western targets from 2003 to 2005.
The militants were crushed inside the kingdom but some fled to Yemen where they set up a new wing of al Qaeda that swore to bring down the Saudi ruling family.