Abdulaziz al-Shubaily and Issa al-Hamid are both members of the local Civil and Political Rights Association (ACPRA), which Saudi authorities banned and shut down in 2013.
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Sources quoted by the Gulf Centre for Human Rights (GCHR) said the arrests were linked to past convictions.
A special court confirmed Shubaily's eight year prison term earlier this year for calling for demonstrations and what authorities described as threatening public order.
Another court sentenced Hamid to 11 years in prison on appeal in 2016, after first being granted a nine years in jail, for "damaging the image of the state".
It was unclear why the two were not serving the sentences for their previous convictions or why they were detained now.
There was no immediate confirmation of the arrests from the government.
"This is a dark time for freedom of expression in Saudi Arabia," Samah Hadid of Amnesty International said in a statement.
"These two arrests have confirmed our fears that the new leadership under Mohammed bin Salman is determined to crush the kingdom's human rights movement," Hadid said, referring to the son of King Salman who became crown prince earlier this year.
GCHR demanded their immediate and unconditional release, calling them prisoners of conscience.
The arrests come after authorities last week detained around two dozen people, including influential clerics, in what activists decried as a coordinated crackdown.
Analysts say many of those detained are resistant to Prince Mohammed's aggressive foreign policy that includes the boycott of Gulf neighbour Qatar as well as some of his bold reforms, including privatising state assets and cutting subsidies.
Saudi officials have instead suggested a foreign plot to overthrow the government, without disclosing details.
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