Human Rights Watch said that "the authorities in Saudi Arabia are trying to silence social and political rights activists and quash their calls for change".
Its new report claims critics have been given travel bans, had their employment terminated, been subjected to smear campaigns, and detained and prosecuted. But they are continuing to voice their concerns, using social media to build networks and launch digital campaigns.
The report's author "Adam Coogle" said social media activity "scares the Saudi authorities", noting that "the Saudi authorities think that by arresting the most prominent activists for their social media activism and human rights activism in general, that they can intimidate everybody else into silence, but really the cat is out of the bag", stressing that "people are talking on social networks now, that are expressing their views and there is really nothing the Saudi authorities are going to be able to do to shut this down".
On Sunday, one activist "Omar al-Saeed" , a member of the Saudi Civil and Political Rights Association (ACPRA) was sentenced to four years in prison and 300 lashes for calling to a constitutional monarchy.
The group said Mr Saeed didn't have proper legal representation at the secret hearing when he was sentenced. Officials did not comment.
Human Rights Watch's report focuses on the plight of 11 prominent activists, including the women's rights campaigner Samar Badawi, who was jailed for speaking out against male guardianship laws and released in April 2011 after a high-profile Twitter campaign.
The government had refused to permit new human rights groups such as the ACPRA, and then sentenced their founders to lengthy prison terms for "setting up an unlicensed organisation", the report said.
A Jeddah lawyer, Waleed Abu al-Khair, and an activist from Eastern Province, Fadhil al-Manasif, are currently on trial on charges including "insulting the judiciary", "trying to distort the reputation of the kingdom", and "inciting public opinion against the state".
In addition to prosecuting activists, the report said the interior ministry regularly banned them from foreign travel for extended periods without providing advance notification or specifying reasons.
Despite such repression, activists were continuing to challenge the authorities, risking their freedom and livelihoods to push for genuine reform and respect for human rights, the report said.
They are using news websites, blogs and social media tools, such as Twitter and Facebook, to build relationships, discuss strategy and develop platforms to disseminate their reform message.
Tens of thousands of Saudi citizens have participated in online campaigns, including the "Women2Drive" initiative, which encourages Saudi women to drive in defiance of a ban.