Gathered around the Shiite village of Daih, men and women waved Bahrain's national flag and held up signs that read: "Manama, capital of torture," the witnesses said.
"Torture is a practice rooted in the security agencies," in Bahrain, the main Shiite opposition bloc Al-Wefaq said in a statement.
It charged that a tug-of-war is underway in Sunni-ruled Bahrain between "a political majority demanding a democratic transition and a hard core dictatorship that refuses any change."
Bahrain, home to the US Navy's Fifth Fleet, was rocked by month-long protests in early 2011 linked to opposition demands for a constitutional monarchy.
Strategically situated across the Gulf from Shiite-ruled Iran, Bahrain, which has a majority Shiite community, has continued to witness sporadic demonstrations, now mostly outside the capital.
Human rights groups say a total of 80 people have been killed in the unrest in Bahrain since February 2011.