Saudi Arabia must "release immediately and unconditionally six Saudis" held for nearly a year in connection with a protest that only one of them attended, Amnesty International said on Friday.
The London-based rights group said five of the men were being held without trial in connection with Saudi Arabia's "Day of Rage" protest which was planned for March 11, 2011.
Khaled al-Johani was thought to be the sole protester on the day, while four other men were detained on the same day and a sixth man was arrested a week earlier, Amnesty said.
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The planned protest, which was called by activists in the wake of the so-called Arab Spring, demanding political reforms in the ultra-conservative kingdom, ultimately did not take place due to the heavy deployment of security forces.
"Holding people for a year merely for intending to protest is completely unconscionable. But that is what it seems the Saudi authorities have been doing in the name of security," said Philip Luther, Amnesty's interim director for the Middle East and North Africa.
The rights group said it had detailed information indicating that at least one of the men was "tortured or otherwise ill-treated after being detained," adding that only Johani had been put on trial so far.
The watchdog said security forces have arrested hundreds of people, particularly in the east of the country, where the Shiite minority are concentrated, for protesting or voicing their opposition to government policies.
Most have been released without charge, but others remain in detention without charge or trial, while several have been charged with vague security-related and other offences, Amnesty added.