Saudi police have arrested 176 people, including 15 women, for holding an illegal protest to demand the release of Islamist prisoners, according to the official SPA news agency.
The agency, quoting a police spokesman, said protesters were arrested "after refusing to break up a gathering outside the offices of the investigation bureau and the prosecution in Buraida," in central Saudi Arabia.
The spokesman accused the protesters of acting on behalf of "deviant groups" -- a term the authorities usually use to refer to the Al-Qaeda jihadist network.
Demonstrations are banned in Saudi Arabia, an ultra-conservative Muslim kingdom that has remained relatively untouched by the Arab Spring uprisings.
Small groups of women have gathered almost daily in Buraida, north of Riyadh, to demand the release of imprisoned Islamist relatives, and dozens of protesters held a rare sit-in outside the Buraida prison in September.
At the time police dispersed the protesters and authorities later warned they would deal "firmly" with demonstrations, sparking condemnation from Amnesty International which urged Riyadh to withdraw its threat.
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On Friday the London-based human rights watchdog criticised the latest arrests.
"This cat and mouse game authorities in Saudi Arabia are playing is, simply, outrageous," said Philip Luther, Amnesty's director for the Middle East and North Africa.
"Instead of persecuting peaceful protesters, what the Saudi authorities should do is listen to their demands and release all those held solely for exercising their human rights."
Amnesty said Friday's protesters were seeking the release of "more than 50 women and children" detained after a similar demonstration two days earlier.
The women and children "were demanding the release of their relatives, incarcerated without charge or trial or beyond the end of their sentences," said the watchdog.
Some of the women also called for the sacking of the interior minister of the country where "criticism of the state is not tolerated," it added.
A wave of deadly Al-Qaeda attacks in the kingdom between 2003 and 2006 prompted a Saudi crackdown that drove out the local branch of the group that was founded by the late Saudi-born Osama bin Laden.
An independent Saudi rights organisation says there are about 30,000 political prisoners in the Gulf kingdom, a charge Riyadh denies, saying there are none.