Abdullah al-Hamed and Rima al-Juraish were seized in the town of Buraida north of Riyadh after security forces said they found a secret weapons stash at Juraish’s home.
A colleague of Juraish, who took part in a women’s protest outside state security headquarters in Buraida on Monday over detentions of their husbands, told Reuters that police planted the weapons themselves to justify arrests.
A statement from London-based Amnesty International said there was reason to believe they were being held over "peaceful exercise of their right to freedom of expression".
"According to reports, they are being held incommunicado and may be at risk of torture or ill-treatment," said the statement sent to Reuters.
The Paris-based Arab Commission for Human Rights said on its Web site: "The Commission strongly condemns this serious escalation and demands the immediate release of the detainees."
Hamed was one of three reformers jailed in 2005 for organising a petition calling for Saudi Arabia, a U.S. ally and the world’s largest oil exporter, to be transformed from an absolute monarchy into a constitutional democracy.
King Abdullah pardoned the men later that year after he ascended the throne of the vast desert kingdom, which has no political parties and bans street protests.
Hamed told Reuters in May that he feared government action to silence him. He was among the organisers of a reform petition this year which was highly critical of the Interior Ministry.
Hamed is the lawyer for Juraish’s husband, one of 3,000 men the authorities admit are being held in detention without charge on suspicion of links to militants.
Al Qaeda-linked Islamists launched a campaign to bring down the monarchy in 2003, targeting government installations, foreigners and the oil industry.
