Amnesty International urged the kingdom on Friday to immediately and unconditionally release all people held "solely for the non-violent expression of their conscientiously held beliefs".
The call came after the Saudis arrested more than 250 people during a protest in Riyadh on 14 October.
The protest took place while a human rights conference organised by the Saudi Arabian Red Crescent was taking place.
The protestors were calling for political reform and the release of political prisoners, despite a ban on anti-government demonstrations in the kingdom.
The Ministry of Interior announced the number of those arrested following the demonstration was 271. It added that 188 were released, but 83 are being interrogated and would be referred for trial.
The Saudi authorities dismissed the protests as the work of marginal groups, but announced those who took part would be punished to prevent similar demonstrations in the future.
However, Amnesty said the Saudi actions were unacceptable.
Saudi Arabia is one of the most
secretive societies on earth
Torture and repression
"The Saudi Arabian government must ensure that any detainee charged with a recognisably criminal offence is given a prompt and fair trial in accordance with international human rights standards," it said.
"Furthermore, detainees must be protected from torture and given regular access to families, lawyers and medical attention if necessary."
Amnesty also said Um Saud, one of three women arrested with the protestors and held in al-Malaz prison in Riyadh, was reportedly beaten and ill-treated during her arrest.
It said it fears that Um Saud and other detainees may be at risk of torture and ill-treatment in detention.
According to Amnesty, critics of the Saudi state are often at risk of indefinite detention without charge or trial.
They are often ill-treated or tortured and detainees do not have the right to formal representation by a lawyer.
"There is a revolutionary spirit in Saudi Arabia. I think the majority of the people are now so fed up with the regime that they see no alternative but to overthrow it"
Committee for the Defence of Legitimate Rights
And due to the secrecy of the Saudi Arabian justice system, trials are often held behind closed doors.
Meanwhile, London-based Saudi dissident Dr Muhammad al-Massari, of the Committee for the Defence of Legitimate Rights, said the protests were only the tip of the iceberg.
He said: "There is a revolutionary spirit in Saudi Arabia. I think the majority of the people are now so fed up with the regime that they see no alternative but to overthrow it.
"The only disagreement lies between those who believe in military overthrow and those who believe in forcing political change through peaceful means."
Saudi officers at the rally called
by London-based Al-Islah
He added: "The Saudis tend to dismiss protests as marginal, but if this is so why are they so afraid to hold elections and allow political activity?"
Saudi security forces were deployed in Riyadh on Thursday, 23 October for a demonstration organised by the London-based Al-Islah (Reform) opposition group, that also called for protests in several parts of the kingdom.
Saudi police detained around 50 people after intervening to stop another demonstration in the Red Sea port city of Jeddah.