Saudi authorities have detained a pro-democracy activist and a woman who led a protest this week over detentions of suspected militants, saying they were being questioned over a weapons stash.

Abdullah al-Hamed, who has been jailed in the past after calling for political reforms, was seized in the city of Buraida, north of Riyadh, on Thursday morning, they said.

He was taken along with Rima al-Juraish and four other women, who took part in a sit-in which Juraish organised on Monday in Buraida, demanding the release of relatives in detention for suspected links to militants, they added.

The four other women were later released.

Hamed is the lawyer for Juraish's husband, who has been in detention without charge for three years on suspicion of links to militants.

The Interior Ministry confirmed the arrests in a statement given to Reuters, but said they took place after a search for weapons at Juraish's home which uncovered three machine guns, three revolvers and some ammunition.

It said Hamed and his brother were arrested for trying to break through a security cordon around Juraish's home. "They refused to obey security instructions and are being questioned to establish his link to the case," the statement said. -Reuters