Saudi security forces shot one person dead and injured three others during what the country's state news agency described as "clashes" in the oil-producing Eastern Province.

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 Awwamiya in Qatif on Friday they were attacked by gunfire," a police statement on the Saudi Press Agency (SPA) said on Friday.

"They dealt with the situation by firing back, which resulted with the death of one."

The SPA also said a protester was killed and three others were wounded in clashes on Thursday.

Activists and witnesses said that security forces opened fire when a Shia procession marking the birthday of the Prophet Mohammad - a celebration forbidden in ultra-conservative Sunni Muslim Saudi Arabia - 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."

Medani's death raises to six the number of protesters killed since demonstrations erupted in the Eastern Province last March against the Saudi-led military intervention to help crush Shia-led pro-democracy protests in neighbouring Bahrain.

Most of Saudi Arabia's estimated two million Shias live in the province, where the vast majority of the kingdom's huge oil reserves lie. They complain of marginalisation in the Sunni-dominated country.

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.