Chanting slogans against the country's absolute monarchy, demonstrators in the cities of Qatif and Awamiyah on Friday also expressed solidarity with anti-government protesters in neighboring Bahrain and condemned Manama's violent crackdown on peaceful protesters.
The protests come despite tight security and a strict ban on all anti-government rallies.
Saudi activists say there are more than 30,000 political prisoners, mostly Prisoner of conscience, in jails across the Kingdom.
According to the activists, most of the detained political thinkers are being held by the government without trials or legitimate charges and they were arrested for merely looking suspicious.
Some of the detainees are reported to be held without trial for more than 16 years.
Attempting to incite the public against the government and the allegiance to foreign entities are usually the ready-made charges against political dissidents.
Families of political prisoners have repeatedly pleaded with the ruling monarchy to at least give their loved ones a fair trial. But for years now, the families say, the king has ignored their calls.
Human Rights Watch says more than 160 dissidents have been arrested since February as part of the Saudi government's crackdown on anti-government protesters.
According to the Saudi-based Human Rights First Society (HRFS), the detainees were subject to both physical and mental torture.