The Shiite notables were rounded up in the kingdom's Eastern Province, where minority Shiites predominate and which is the center of Saudi Arabia's oil industry.
It's thus seen as highly vulnerable by the country's Sunni rulers, particularly to destabilization by Shiite-dominated Iran.
Tehran denies it's spying. But both nations have for years been engaged in a covert war as they vie for supremacy in the Persian Gulf and across the Middle East.
That religion-based conflict, which dates back to Islam's early days in the 8th century, has escalated in recent years, particularly since Iran was bolstered by the empowerment of neighboring Iraq's Shiite majority, courtesy of the United States, and Tehran's expansionist ambitions.
The roundup of Shiite notables in Eastern Province followed the arrest of the alleged spies -- 16 Saudis, a Lebanese and an Iranian -- which Riyadh announced March 19.
The Saudis have blamed a wave of recent unrest, in which Shiites protested against repression, marginalization and discrimination by Riyadh, on Iranian agitation at a time when the monarchy is struggling to stifle dissent amid the pro-democracy uprisings across the Arab world.
The arrest of the alleged spies touched off a wave of anti-Shiite rhetoric in Saudi Arabia's state-controlled media. Shiite leaders said the accusations of espionage were intended to distract a population hungry for reform.
Amid rising tension between the monarchy and the kingdom's Shiite minority, the authorities have summoned scores of Shiite notables, mainly from the Eastern Province towns of Ihsaa and Qatif, for interrogation, Saudi activists say.
This, they contend, has heightened the tension.
Among the notables was Sheikh Hassan al-Saffar, an influential Shiite cleric who led the minority's political movement in exile before he returned to Saudi Arabia in the 1990s after reconciling with the late King Fahd.
He was interrogated after he questioned in a sermon whether there really was an Iranian spy ring, thereby directly challenging Riyadh, and demanded equal rights and privileges for Shiites.
"The days when the people were silent are over," he declared. "They won't accept any more to be kept in the dark with no participation in decision making, or real representation, fighting corruption, or holding prisoners without trial."
Meantime, the Paris-based website Intelligence Online, reports that a key element in the escalating the Saudi crackdown was the Aug. 15, 2012, cyberattack on state-owned Saudi Aramco, the world's biggest oil exporter with reserves of 296 billion barrels.
That attack, using a virus that investigators dubbed "Shamoon," wiped out at least 30,000 computers at Aramco, and erased data on the domain management servers that are the heart of the corporate network.
Riyadh blamed it on Iran, whose national oil company was hit by a virus in April 2012, probably as part of a U.S.-Israeli cyber offensive against the Islamic Republic and its nuclear program.
The Aramco sabotage was supposedly carried out by at least one company employee with access to the computer system, and probably was linked to Saudi Shiite activists in the Eastern Province where the kingdom's oil industry's centered.
Intelligence Online says Aramco "narrowly escaped being the target of new attacks" in the weeks that followed, but gave no details.
The website said Prince Bandar bin Sultan, director of the General Intelligence Presidency, and Prince Mohammed bin Nayef, the interior minister, reported to King Abdallah recently on "a four-month probe that culminated in the recent arrests."
Intelligence Online said the alleged Iranian network "made up mostly of Shiites, had focused on Aramco's facilities, particularly the control systems of the Ghawar oil field ... and the Ras Tanura refinery."
Ghawar, in the east of the kingdom, is the largest conventional oil field in the world. It produces 5 million barrels per day, and since its discovery in 1948 it has produced more than 65 billion barrels of oil.
Ras Tanura, on the gulf coast, is the world's largest crude export refinery.
Riyadh's worst nightmare has been that the Shiites in Eastern Province rise up.
Knocking out these facilities, which al-Qaida has tried and failed to do, would seriously damage Saudi Arabia's oil output and export capability.