The arrests took place over the last two days, mainly in Al-Qatif and neighbouring towns which were the scenes of protests calling for the liberation of political prisoners and in solidarity with the opposition in Bahrain.
Bloggers Mustapha al-Moubarak and Hussein al-Hashem were arrested at the beginning of the week, according to rasid.com site, which claims to have the names of 25 detainees.
Another 100 people have been in detention since their arrest at the beginning of march when demonstrations first broke out.
The majority of Saudi Arabia’s two million Shia live in the eastern province, next door to Bahrain, which is ruled by a Sunni royal family although the majority of its population is Shia.
Saudi Arabia sent troops to Bahrain to help tackle protests there last month.