Salman al-Odah was arrested alongside preacher Awad al-Qarni. Both have large social media followings (Flickr)
A Saudi cleric has been imprisoned without charge for the past four months after refusing to publicly support the blockade on Qatar, Human Rights Watch said.
The rights group on Sunday said that local authorities had also imposed travel bans on 17 members of Salman al-Odah's family after he was detained in September 2017.
A family member told HRW that Odah was being held in solitary confinement because he refused to comply with an order to tweet out support for the Saudi-led blockade of Qatar.
He instead opted for a tweet calling on reconciliation between the different countries: "May God harmonise between their hearts for the good of their people."
Saudi Arabia, the United Arab Emirates, Bahrain, and Egypt imposed a blockade on Qatar in June, accusing Doha of supporting "terrorists" and having close ties with Iran. Qatar denies the allegations.
Family members said that two men who arrested Odah identified themselves as state security, searched his house, and ignored repeated requests to show a warrant.
Odah is among dozens of dissidents, writers, and scholars detained in mid-September as part of a crackdown against those acting "for the benefit of foreign parties against the security of the kingdom and its interests".
HRW's Middle East director Sarah Leah Whitson said that Saudi Crown Prince Mohammed Bin Salman's economic and societal reforms are bound to fail if Riyadh continues "arbitrary arrests" of any opposition.
"If Mohammed bin Salman wants to show that a new era has begun in Saudi Arabia, a refreshing first step would be the release of activists and dissidents who have never been charged with a recognisable crime and should never have gone to jail in the first place," said Whitson.
Activists have circulated lists of more than 60 people being held, though Saudi authorities have not released information about their cases.
Other prominent detainees in the group arrested include economist Essam al-Zamil, academic Mustafa al-Hasan, writer Abdullah al-Malki and dozens of other scholars.
Odah's brother Khaled was also arrested after he tweeted about his brother's detention and he continues to be held by Saudi authorities.
Riyadh carried out another wave of arrests in early November of Saudi princes and prominent businessmen in a bid to allegedly clamp down on corruption in the Gulf Kingdom.
Saudi courts have convicted at least 25 prominent activists and dissidents since 2011. Many faced sentences as long as 10 or 15 years, under charges designed to criminalise.