Dozens of police cars filled the Olaya commercial centre in Riyadh, where the protesters are supposed to congregate. Police patrols are checking the identities of motorists.
The activists have urged Saudis to demonstrate in large numbers soon after the Muslim Friday prayers around 1000 GMT in most of Saudi cities including Riyadh, Jeddah and Dammam.
Online activists using Facebook and Twitter have called for a "Day of Rage" to campaign for a fully elected parliament and ruler.
The activists are demanding an independent judiciary, the abolition of the secret service police, the release of all political prisoners, guarantees of freedom of expression, a fixed minimum wage of 10,000 riyals (two euros). They also want jobs – the unemployment rate is 10.5 per cent, and in the 20-29 age group, it is 30 per cent, according to official figures.
Last month, King Abdullah decreed benefits to Saudis estimated at more than 20 billion euros, which included housing services and unemployment benefits.
On Thursday, police shot and injured three protesters in Al-Qateef in the eastern province. Officials say the police fired in self-defence.
The Saudi interior ministry had earlier issued a reminder that demonstrations are illegal and warned activists that the security forces had been authorised to crack down on any protests.
The United States said it would closely monitor unrest in Saudi Arabia, the world's biggest oil exporter.
Political parties are banned in Saudi Arabia, which controls a quarter of the world's oil reserves.
Saudi Foreign Minister Prince Saud al-Faisal said on Monday the kingdom "completely rejects any intervention in its internal affairs," and warned it will "chop off the fingers," of those trying to meddle in Saudi affairs.