Activists contacted by AFP from Dubai said that Zuhair Al-Said, 21, was killed as security forces dispersed a protest on Friday against the death of another Shiite demonstrator the previous day. "Security forces following an unauthorised gathering in the (Shiite) town of Al-Awamiya in Qatif district came under fire from masked gunmen," SPA quoted a police spokesman as saying.
Police "responded, sparking an exchange of fire that resulted in the wounding of one of them, who died later."
An activist told AFP that Said was "shot dead by security forces as they dispersed a protest against the killing of another man" on Thursday.
"Eight armoured vehicles belonging to Saudi security forces intervened to disperse the protest," the activist said.
Munir al-Medani, also 21, died of his wounds on Thursday after being shot by security forces in the Al-Shwaika neighbourhood of Qatif during a Shiite demonstration for reform in the ultra-conservative Sunni Muslim kingdom, activists said.
Saudi authorities said that Medani too died in an exchange of fire between security forces and "masked men."
Demonstrators also took to the streets of another Qatif town -- Al-Rabieya -- on Friday to protest against Medani's death, activists said. "Hundreds of demonstrators waved pictures of those killed and detained as they condemned the shooting (by Saudi security forces) on peaceful protests," one activist told AFP.
Activists and witnesses said that Medani's death came when security forces opened fire on a Shiite procession marking the birthday of the Prophet Mohammed -- a celebration forbidden in Saudi Arabia -- which turned into a demonstration for reform and the release of Shiite detainees.
Said's death raises to seven the number of protesters killed since demonstrations erupted in the Eastern Province last March.
The immediate trigger for the protest movement was a Saudi-led military intervention in neighbouring Bahrain to help crush Shiite-led pro-democracy demonstrations.
Activists say that Saudi authorities have arrested nearly 500 people since the protests started. Many have been released but dozens remain in custody, among them human rights activist Fadel al-Munasif and writer Nazir al-Majid.
In January, Saudi authorities published a list of 23 men wanted on suspicion of involvement in the disturbances.
Later the same month, the interior ministry announced that security forces had arrested nine people suspected of involvement in the wounding of three policemen in the Eastern Province.
Most of Saudi Arabia's estimated two million Shiites live in the province, where the vast majority of the kingdom's huge oil reserves lie. They complain of marginalisation in the Sunni-dominated kingdom.