Saudi authorities arrested a university professor a day after he called for the release of jailed relatives and other prisoners, a Saudi human rights group said on Wednesday.
Saudi Arabia, the world's top oil exporter and a key U.S. ally, is an absolute monarchy that does not tolerate any form of public dissent.
"Dr. Mubbarak (bin Zuair) was supposed to break the good news to the demonstrators in front of the Ministry of Information who were protesting the extended illegal detention of their loved ones, that some of the detainees would be released," the Human Rights First Society (HRFS) said in a statement.
"At 10.30 am on March 20, on his way to the Ministry of Information where the standoff was taking place, Dr Mubbarak was stopped and arrested by the secret police," it added.
Interior Ministry spokesman Mansour al-Turki could not confirm the arrest of bin Zuair, a professor at Alimmam Mohammad Bin Saud University in Riyadh.
On March 19 bin Zuair met with the assistant secretary for security affairs at the ministry to ask for the release of his relatives and other prisoners.
His father, Professor Said bin Zuair, an Islamist and outspoken critic of the Saudi royal family, has been imprisoned without trial for around five years, the group's president Ibrahim al-Mugaiteeb told Reuters.
His brother, Saad bin Zuair, a lawyer, has also been held without trial for around four years, he said.
"HRFS was appalled and astonished by his illegal arrest," the group said in the statement, calling for his and his relatives' release.
Dozens of Saudi men gathered outside the Interior Ministry in the capital Riyadh on March 20, to demand the release of jailed relatives.
Saudi Arabia, an absolute monarchy with no elected parliament, is printing 1.5 million copies of an edict by religious scholars outlawing protests in the conservative kingdom as "un-Islamic", the state news agency said on Tuesday.
The kingdom has avoided the mass protests which toppled longstanding leaders in Egypt and Tunisia and spread to the Arabian Peninsula, affecting Bahrain and Yemen and to a lesser extent Oman and Kuwait.
It managed to stifle an attempt to stage a mass protest on March 11 with a large security presence on the street.
Shi'ite protesters have demonstrated in the kingdom's oil-producing east, calling for the release of prisoners and the withdrawal of Saudi forces from Bahrain.
In a joint statement, the G7 leaders of Britain, Canada, France, Germany, Italy, Japan, and the United States, along with the European Union, said they "are united in rejecting the electoral process" that led to the May 20 ballot.
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