On Friday, one peaceful demonstrator was martyred and three others were injured when Saudi security forces opened fire on protesters in the town of Awamiyah, after the Friday Prayers leader demanded that the Saudi monarchy be abolished.
The kingdom's state news agency said the peaceful rally was illegal.
Activists and witnesses said Friday's casualties came when security forces opened fire on an anti-government demonstration in the Qatif district.
"While security men were following up on an illegal gathering in the town of Awamiyah in Qatif on Friday, they were attacked by gunfire," a police statement on the Saudi Press Agency (SPA) said.
Activists and witnesses said that security forces opened fire when a procession marking the birthday of the Prophet Mohammad (PBUH) turned into a demonstration for reform and the release of detainees.
"Munir al-Medani, 21, was wounded by a live bullet to his chest," one activist told the AFP news agency, requesting anonymity. "He was taken to hospital where he later died of his wounds."
"The youth Zaheer Abdullah Saeed also wounded by a live bullet to his waist, he was taken to hospital where he later died of his wounds in Qatif.
Zaheer Abdullah Saeed's martyrdom raises to six the number of protesters martyred since demonstrations erupted in the Eastern Province last March against the Saudi-led military intervention to help crush pro-democracy protests in neighboring Bahrain.
Anti-regime demonstrations were also held in the Eastern Province towns of Qatif, Tarut, and Rabi’iyah.
The incidents occurred one day after another anti-regime protester was killed and 14 people were seriously injured in Qatif.
Several of those injured in the deadly crackdown of the past two days are in a critical condition.
There have been demonstrations in Saudi Arabia's Eastern Province on an almost daily basis over the past few months, with protesters calling for political reform.
Anti-government protests have intensified since November 2011, when security forces opened fire on protesters in Qatif, killing five people and leaving scores more injured.
Activists say there are over 30,000 political prisoners in Saudi Arabia.
Saudi activists declared Saturday a day of public mourning.
Schools and shops remained closed as protesters staged demonstrations in the eastern region.
People took to the streets in the cities of Qatif, Awamiyah and Safwa on Saturday, calling for an immediate regime change.
Saudi forces reportedly opened fire on peaceful demonstrators, who also condemned the monarchy’s deadly crackdown on peaceful protests.