Hundreds of police have been deployed in the Saudi capital, Riyadh, ahead of anti-government rallies planned for after Friday prayers.
Security forces have blocked roads and set up checkpoints, while reports suggest some protesters have begun to gather in the eastern town of Hofuf.
On Thursday, police opened fire at a rally in the eastern city of Qatif, with at least one person being injured.
Activists have been inspired by a wave of popular revolt across the region.
Analysts have warned that any violence could have a big impact on global oil prices.
Protests are illegal in Saudi Arabia, which has had an absolute monarchy since its unification in the 1930s.
Interior ministry spokesman General Mansor al-Turki has told the BBC that the situation is under control and police are not using excessive force.
Last month, the arrest of Shia cleric Sheikh Tawfiq al-Amer, detained reportedly for calling for a constitutional monarchy, sparked outrage and drew crowds on to the streets.
He was released last weekend, but relatively small-scale protests have continued in the Eastern Region, where much of the country's crude oil is sourced.
On Thursday, an interior ministry spokesman told reporters that police had fired over the heads of protesters during a rally in Qatif, while witnesses said police also beat demonstrators with batons.
The ministry spokesman added that three people, including a policeman, had been injured.
The protesters have been demanding the release of nine Shia prisoners who they say have been held without trial for more than 14 years.
Shias, who are mainly concentrated in the east of the country, make up about 10% of the population in Sunni-dominated Saudi Arabia.
The region borders Bahrain, a Shia-majority kingdom ruled by a Sunni government that has been rocked by anti-government protests since mid-February.
Amid signs of growing unrest in the region, Saudi Arabia's King Abdullah recently unveiled $37bn (£22.7bn) in benefits for citizens, including a 15% pay rise for state employees.