(CNN) -- A Saudi Arabian political science professor who is an outspoken human rights advocate was taken into custody this week by the country's secret police, his wife said Friday.
Matrook al-Faleh, shown in 2004, was seized after he criticized prison conditions, says Human Rights Watch.
Human Rights Watch issued a report calling for the release of Matrook al-Faleh, 54, who was seized at King Saud University in Riyadh, Saudi Arabia, two days after he publicly criticized conditions in a prison where two Saudi human rights activists are serving jail terms, according to the agency.
Jamila al-Uqla, al-Faleh's wife, said her husband went to the university on Monday and never returned. She said police informed her that he was in custody at the city's main detention facility.
The next day, al-Uqla said, she tried to call police back, but no one could give her more details. She said that police did not say why he was seized and that she has not heard from him since.
"I keep calling the secret police, but they keep denying they're holding him at their facility," she said. "I have called his cell phone, but there is no answer. He has not called even once."
Al-Uqla said she and al-Faleh are patriotic Saudis.
"My husband is transparent and doesn't hide anything. He says whatever he sees. He has loyalty to his country and the interests of his country," she said.
Joe Stork, deputy director at Human Rights Watch's Middle East division, said the arrest shows that human rights advocacy in Saudi Arabia remains "a risky business."
"By suppressing peaceful dissent, Saudi Arabia only stands to gain further notoriety as an abuser of human rights," Stock said in the report.
Phone calls to Saudi Arabia's Interior Ministry were not answered.
On Saturday, Human Rights Watch said that al-Faleh sent an e-mail to human rights activists and journalists about visiting procedures and detention conditions at a Buraida General Prison, where his fellow friends are being held.
He called the visiting procedures laborious and compared the visiting area to a "chicken coop."
"Al-Faleh's fellow activists, the brothers Abdullah al-Hamid and Isa al-Hamid, are serving prison sentences at Buraida General Prison for expressing support for a demonstration that took place in front of Buraida's secret police prison by wives and relatives of long-term detainees held there without charge," the group said.
The activists decried the prison's conditions as filthy and crowded, with poor health care, according to Human Rights Watch, which said it independently confirmed such conditions.
"It is outrageous that the Ministry of Interior arbitrarily arrests Dr. al-Faleh rather than addressing the inhumane conditions he documented," Stork said.
Al-Faleh, Abdullah al-Hamid and Ali al-Dumaini, who runs a Saudi discussion site, were arrested in 2004 for circulating a petition to then-Crown Prince Abdullah calling for a constitution guaranteeing basic human rights.
A court sentenced Al-Faleh, Abdullah al-Hamid and Ali al-Dumaini, to six, seven and nine years respectively, but King Abdullah pardoned them in August 2005, Human Rights Watch said.
Al-Faleh's recent statement had been reproduced on menber-alhewar.info, al-Dumaini's Web site, according to Human Rights Watch.
In contrast to her husband's detention on Monday, al-Uqla said police in 2004 allowed her husband to call her just after his arrest.
Saudi authorities arrested blogger Fouad al-Farhan in December after he called for the release of a group of detained peaceful reform activities. He was released in April.
CNN's Roba Alhenawi, Mohammed Jamjoom and Joe Sterling contributed to this report.
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