Amnesty International urged Saudi Arabia to free Shiite prisoners arrested for taking part in "peaceful" protests and those detained without charge in the kingdom's Eastern province.
In a report called "Dissident Voices Stifled in the Eastern Province," where most of the country's minority Shiites live, the rights group on Monday said hundreds of individuals, mainly men, but also children, had been arrested since March 2011.
Though "many of those arrested have been released some continue to be detained, mostly without charge or trial and apparently solely for exercising their right to freedom of expression," according to the report.
Saudi's minority Shiites first took to the streets in protest in February 2011 after an outbreak of violence between Shiite pilgrims and religious police in the holy city of Medina.
The protests escalated after the kingdom led a force of Gulf troops into neighbouring Bahrain to help crush a month-long Shiite-led uprising against the country's Sunni monarchy.
The report singled out the cases of nine men and a 16-year-old who remained in custody. Seven of the 10 detainees were arrested more than six months ago and had yet to be "informed of any charges filed against them," it said.
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According to Amnesty, "none of these detainees has been allowed to appoint a lawyer (except for one)... and none of them have been allowed to challenge the legality of their continued detention."
The London-based rights group also accused Riyadh of "torture or other ill-treatment" of prisoners and said public- and private-sector employees "who choose to exercise their rights to freedom of expression... risk losing their jobs."
Saudi authorities have "blocked several websites operating from the Eastern province," according to the report, apparently for "posting news about demonstrations and other activities opposed to government policies."
Amnesty said it was concerned about the "use of force" by security forces in their clampdown on Shiite protests, adding that since November 21, "seven men have died and a number of others injured after being shot while protesting".
Saudi authorities have said the deaths were the result of clashes between police and armed individuals.
The report said the measures taken by Saudi authorities amount to a "pattern of widespread human rights violations" against residents of the province.
Oil-rich Eastern province is home to some two millions Saudi Shiites who for decades have complained of marginalisation by the Sunni monarchy.
On May 16, Amnesty submitted a "memorandum" to the Saudi government citing the alleged violations and concerns raised in the report but said it has so far "not received a response."