At least one person has been killed and three others injured in clashes between security forces and Shia protesters in eastern Saudi Arabia, activists say.
Issam Mohammed, 22, reportedly died when troops fired live ammunition after demonstrators threw stones at them in al-Awamiya, a town in the Qatif region.
Officials said a security vehicle was shot at and attacked with petrol bombs.
The violence came as UK Prime Minister David Cameron arrived in Saudi Arabia for his first official visit.
Mr Cameron will meet King Abdullah and Crown Prince Nayef, the interior minister, to discuss the strengthening of security, trade and energy ties with the UK, growing tensions with Iran and the civil unrest in Syria.
Saudi investment in the UK is worth more than £62bn ($95bn) and the Gulf kingdom is the UK's biggest trading partner in the Middle East.
The Saudi Shia news website Rasid.com said Mr Mohammed had suffered gunshot wounds when security forces opened fire to disperse protesters in al-Awamiya at dawn on Friday, in retaliation for stones being thrown at a security vehicle.
One of the three injured was shot while trying to drive through a checkpoint at the entrance to the town, the website added. Security forces reportedly sealed off the town after the clashes.
The interior ministry said security forces patrolling al-Awamiya in a vehicle had been attacked with petrol bombs and had caught fire.
"While the security forces were trying to control the fire, they were exposed to shooting and were dealing with the situation as necessary," a statement quoted by state news agency SPA said.
"The exchange of fire led to two people being injured among those involved in the shooting, and they were taken to hospital where one of them died later," it added.
The clashes came after demonstrations were held in four villages in the Qatif region to call for the "release of political detainees, reform and an end to sectarian discrimination", one activist told the AFP news agency.
Qatif, in the oil-rich Eastern Province, is home to a Shia majority that has long complained of marginalisation at the hands of the Sunni ruling family, the Al Saud.
Protests erupted in Eastern Province in March when the popular uprising in neighbouring Bahrain, which has a Shia majority and a Sunni royal family, was crushed with the assistance of Saudi and other Gulf troops.
In November, four Shia men were shot dead by security forces over four days in the city of Qatif. The interior ministry said they had been armed and operating on "foreign orders" - code for Iran.
Earlier this month, the authorities named 23 suspects in connection with the disturbances in Eastern province, accusing them of possessing illegal weapons and opening fire on the public and police.
About 400 people have been arrested since March, of whom 70 remain in custody, according to activists, including the author Nazir al-Majid and the human rights activist Fadil al-Munasif.