Several hundred people have protested in Shia eastern Saudi Arabia but hundreds of police prevented protests in the capital calling for democratic reforms.
Police blocked roads and set up random checkpoints in Riyadh, searching residents and vehicles around a central mosque as large numbers of people gathered for Friday prayers.
Witnesses said groups of policemen manned street corners and intersections and a helicopter flew over the city.
By midday local time on Friday, no protesters had shown up in the capital and the police presence significantly decreased.
In the eastern city of Qatif and nearby areas where the country's minority Shias live, several hundred people staged protests, shouting slogans calling for reforms and equality between Shia and Sunnis.
In Qatif, the protesters were surrounded by armoured personnel carriers and dozens of riot police in full gear.
On Thursday, violence broke out at another protest in Qatif, when Saudi police opened fire to disperse demonstrators. At least three protesters and one police officer were wounded. Friday's protest was largely peaceful.
Activists have been emboldened by other uprisings in the region that have toppled long-time rulers of Tunisia and Egypt. The Saudi activists have set up online groups calling for protests in Riyadh.
Security officials on Friday said security measures around state-run oil giant Saudi Aramco and its oil facilities in the east were beefed up protectively, in case of any violence. The company is based in Dhahran district on the kingdom's eastern coast.
The pro-Western monarchy is concerned protests could open footholds for Shia powerhouse Iran and has accused foreigners of stoking the protests, which are officially forbidden.