Saudi security forces apprehended top Shiite cleric Sheikh Jafar Sweileh on political grounds in Qatif, the Arabic-language al-Ahed news website quoted peace activists as saying on Friday.
Acceding to the report, activists say that the cleric has been arrested for his writings in defense of freedom of expression.
Since February 2011, the Shiite-dominated Eastern Province has been the scene of peaceful protests, mostly in the districts of Qatif and Awamiyah.
The demonstrators have been demanding reforms, freedom of expression and the release of political prisoners. They also want an end to economic and religious discrimination against the oil-rich district.
Several demonstrators have been killed and dozens of activists arrested since the beginning of protests in the region.
International human rights bodies, including Amnesty International, have criticized Saudi Arabia for widespread and systematic violations of human rights.