RIYADH (AFP) – Saudi Arabia launched a massive security operation Friday in a menacing show of force to deter protesters from a planned "Day of Rage" to press for democratic reform in the conservative kingdom.

Illegal demonstrations were supposed to start after Muslim Friday prayers at noon but as the mosques emptied there were no signs of rallies, with security forces manning checkpoints in key locations across several cities.

In the strategic Olaya commercial centre of the capital Riyadh, where protesters were urged to congregate, hundreds of security personnel surrounded the mosque and inspected motorists' identification documents.

Clerics sermonised against demonstrations, saying public agitation was unjustified under Islamic teachings and would only spread chaos.

Online activists using Facebook and Twitter had called for a "Day of Rage" and a "Saudi March 11 revolution" demanding a fully elected parliament and ruler in this conservative Islamic monarchy.

But the information ministry took journalists on a tour of Riyadh which revealed only a large security presence and no protests.

The Red Sea port city of Jeddah, the second biggest Saudi city, was also calm with a significant numbers of police on the streets.

Tensions were high in the Eastern Province city of Al-Qateef, where three Shiite protesters were shot and wounded by police dispersing a demonstration late Thursday.

The shooting happened when around 600-800 protesters, all Shiite and including women, took to the streets of the city of Al-Qateef to demand the release of nine Shiite prisoners, said a witness, requesting anonymity.

A small demonstration calling for reforms and the release of Shiite prisoners also took place Friday, a witness told AFP, but there were no reports of unrest.

Saudi Arabia, with about a quarter of the world's oil reserves, is a linchpin of security in the Middle East and signs of instability in the kingdom are nervously monitored by the United States and other powers.

So far it has been spared the political tumult that has gripped neighbouring countries like Yemen and swept the autocratic leaders of Egypt and Tunisia from power.

Human rights activist Fuad al-Farhan said the Saudi people wanted change but were "not ready to revolt."

"This explains why calls for protests failed miserably," he said.

Responding to Thursday's incident, the United States said it was keeping an eye on the situation and restated its support for universal values of human rights.

Australia warned its citizens in the country to "avoid all protests as they may turn violent."

Activists are demanding far-reaching reform including representative government, an independent judiciary, the abolition of the secret police, the release of all political prisoners and guarantees of freedom of expression.

On the economic front, they seek a minimum monthly wage of 10,000 riyals ($2,667) and jobs for Saudis, in a country where the unemployment rate is 10.5 percent and soars to around 30 percent in the 20-29 age group.

Other Facebook activists have called for nationwide protests on March 20.

On his return from surgery in the United States last month, King Abdullah decreed benefits estimated at more than $30 billion such as housing services and unemployment payments.

Disquiet has stemmed in part from the unequal distribution of the country's oil riches, with about 40 percent of the population living in relative poverty.

Crude oil futures slumped as protests failed to materialise in Saudi Arabia and traders bet that a massive earthquake in Japan would slash the country's crude imports.

New York's main contract, light sweet crude for April delivery, fell to $99.01 -- the lowest level since February 25. It later stood at $99.69 a barrel, down $3.01 from Thursday's closing level.