A Saudi policeman and an armed protester were killed in overnight clashes in the kingdom's oil-rich, Shiite-populated Eastern Province, the interior ministry said on Saturday.

"A security patrol came under heavy gunfire from four armed rioters on motorbikes" in the Shiite district of Qatif, ministry spokesman Mansur al-Turki said, quoted by official news agency SPA.

Policeman Hussein Zabani was killed while another, Saad al-Shummari, was wounded and hospitalised, he said.

"Security forces hunted down the armed rioters who were on motorbikes and exchanged fire with them," he said. "One of the four caught was wounded and he died while on the way to the hospital."

Witnesses said the attackers were participating in a demonstration that took place in Qatif late on Friday.

The demonstration, to mark a Shiite religious celebration, turned violent and police forces opened fire on protesters, wounding at least two, the witnesses said.

One of them was killed, they said, identifying the man as 18-year-old Hussein al-Kallaf.

Confrontations have intensified recently between police and protesters from the Sunni-dominated kingdom's marginalised Shiites -- estimated at about two million and mostly concentrated in Eastern Province.

Two Shiite protesters were killed last month, triggering attacks on government buildings in Qatif.

In May, Amnesty International said seven people had been killed and a number of others injured in clashes between the authorities and protesters in the region since November.

Qatif witnessed a spate of demonstrations after an outbreak of violence between Shiite pilgrims and religious police in the Muslim holy city of Medina in February 2011.

The protests escalated when the kingdom led a force of Gulf troops into neighbouring Bahrain the following month to help crush a Shiite-led uprising against the Sunni monarchy.