Sheikh Salman al-Awdah, an influential cleric who was imprisoned from 1994-99 for agitating for political change and has 14mn followers on Twitter, appears to have been detained over the weekend, the posting suggested.
In one of his last postings on Twitter, he welcomed a report on Friday suggesting that a three-month-old row between Qatar and four Arab countries led by Saudi Arabia may be resolved.
"May God harmonise between their hearts for the good of their people," Awdah said on Twitter after a report of a telephone call between His Highness the Emir Sheikh Tamim bin Hamad al-Thani and Saudi Crown Prince Mohamed bin Salman to discuss ways to resolve the rift which began in June.
Hopes for a breakthrough were quickly dashed when Saudi Arabia suspended any dialogue with Qatar, falsely accusing it of "distorting facts".
Saudi Arabia, along with the United Arab Emirates, Bahrain and Egypt accuse Qatar of supporting Islamist militants, a charge Doha denies.
Al-Awdah was the second cleric reported detained by Saudi authorities in the past week. Reports on social media said that Awad al-Qarni, another prominent cleric with 2.2mn Twitter followers, was also detained from his home in Abha in southern Saudi Arabia.
Like Awdah, Qarni had also expressed support for reconciliation between Arab countries and Qatar.
Saudi officials could not immediately be reached for a comment on the reported arrests.
The al-Saud family has always regarded Islamist groups as the biggest internal threat to its rule over a country where appeals to religious sentiment can never be lightly dismissed and where Islamist militants have previously targeted the state.
A decade ago it fought off an Al Qaeda campaign of attacks targeting officials and foreigners that killed hundreds. In the 1990s, the Sahwa (Awakening) movement inspired by the Muslim Brotherhood demanded political reforms that would have weakened the ruling family.
Reports of the arrests coincided with widespread speculation, dismissed by officials, that King Salman intends to abdicate in favour of his son Crown Prince Mohamed.
Asked about the reasons for the arrests, a Saudi analyst speculated: "(To) crush the Muslim Brotherhood or scare others if their plan is for him (Crown Prince Mohamed) to be king."
Exiled Saudi opposition activists have called for protests on September 15 intended to galvanise opposition to the royal family.
Sheikh Salman al-Awdah (L), Awad al-Qarni
A prominent Saudi religious leader has been arrested, according to social media postings on Sunday, in what appears to be a crackdown on Islamists seen as critics of the conservative kingdom's absolute rulers.
