MANAMA, BAHRAIN -- A young Shiite Moslem rattles through the back streets of this island capital where thousands of U.S. troops have arrived to defend the Persian Gulf's rich oil fields. The Shiite has agreed to show a visitor the underside of the gulf's oil boom, but as he drives he keeps a close eye on his rear-view mirror, worried that he is being followed by security police.

When a suspicious Mercedes-Benz trails him into a Shiite slum quarter, he doubles back to shake it off. As he drives, he lists Shiite grievances against the sheiks and princes of Saudi Arabia and the smaller gulf kingdoms, ruled for decades by Sunni Moslems and now bolstered by a massive, U.S.-led military buildup.

Although he worked for years as a welder in the nearby Saudi oil fields, he has never been granted a passport, not even to travel back and forth to work, the Shiite said. Job opportunities are limited, and fear is pervasive. Shiite activists here and in Saudi Arabia's Eastern Province are sometimes arrested, detained without trial and, according to the London-based human rights organization Amnesty International, subjected to physical torture.

"I am a prisoner," he said, adding, "There is no trust" between Shiites in the gulf and the Sunni sheiks who hold the region's wealth and power.

"The American government should think about who it is supporting. Who are these rulers and what about the rest of the people?" he asked.

Such questions from the Shiite minority in the gulf raise a troubling problem for the United States as it seeks to protect the tens of thousands of U.S. troops in Saudi Arabia and surrounding sheikdoms. Just as they did eight years ago in Lebanon, U.S. forces here have taken up positions amid a large population of Shiite Moslems who have sympathies for revolutionary Iran and a sometimes powerful sense of grievance about their own conditions.

Saudi and gulf kingdom officials appear confident that the disaster of Lebanon -- where 241 U.S. servicemen died in a 1983 truck bomb attack mounted by pro-Iranian Shiite terrorists -- will not be repeated here. They say their own Shiite populations are far more stable and prosperous than the one in Lebanon, and they add that security forces have clipped the wings of pro-Iranian troublemakers in the gulf.

Most independent analysts agree that the chance of widespread Shiite unrest here is small. In contrast to the anarchy of Lebanon, Saudi Arabia and the smaller gulf states are wealthy, tightly ordered societies with strong internal security forces.

"You can see how safe Americans are, whether civilians or in uniform," said a senior Bahrain government official. About fears of Shiite discontent, he said, "I don't think it's an issue."

This official added, "There is a feeling that the people are collectively supporting each other and facing a real danger. I assure you this is the feeling of everybody in this area."

Still, U.S. officials are clearly concerned -- since it would take only a handful of terrorists to mount a devastating attack. Iran's supreme Shiite religious leader, Ayatollah Ali Khamenei, said this month that a "holy war" against U.S. troops in the gulf was justified and that anyone who died in an attack on U.S. forces would be counted as a martyr under Moslem religious law. That prompted a sharp reply from Secretary of State James A. Baker III as well as stepped-up efforts to win Iranian support for the U.S.-led campaign to force Iraq's withdrawal from Kuwait.

The issue of potential Shiite activism in Saudi Arabia and the gulf highlights the perilous complexities of a strategically vital but unevenly prosperous region where small royal clans wield virtually absolute control over social and political life.

The grievances of local Shiites against their ruling Sunni sheiks are rooted in a centuries-old split in the Islamic world. Shiite Moslems believe that after the death of the prophet Mohammed in the 7th century, Sunni Moslems stole the mantle of Islam from its proper inheritor, Imam Hussein, the prophet's grandson. Shiite religious beliefs have long inculcated a spirit of revenge against Sunnis for their perceived transgressions.

Shiite Iran's radical Islamic revolution in 1979 revitalized Shiite grievances in the Middle East. During the 1980s, Iran's government offered material support to Shiite terrorist groups seeking to destabilize Sunni regimes or to attack Western targets. In 1981, pro-Iranian Shiites led a failed coup attempt against the Sunni royal family in Bahrain, a small island nation that is the headquarters for the U.S. Navy command ship in the Middle East and the host of a large but undisclosed number of U.S. forces.

But the Shiite population that perhaps most worried Western planners prior to the present gulf crisis lives in the nearby Eastern Province of Saudi Arabia, clustered in ramshackle colonies amid the oil fields that hold about 20 percent of the world's proven reserves.

Nobody knows exactly how many Shiites live in Saudi Arabia, partly because the government is sensitive about its minority and discourages a comprehensive census, but the number is far from trivial. While Sunnis form the vast majority of the overall population, Shiites may comprise up to half of the Eastern Province's native residents, according to estimates by Western specialists.

Shiite activists in Saudi Arabia claim that members of their community are treated as second-class citizens, deprived of economic opportunities, harassed arbitrarily by security forces and prevented from publicly observing Shiite religious festivals.

Following unrest in Shiite areas of the Eastern Province in the late 1970s and early 1980s, the Saudi government shifted gears and began to pour development funds into the area, hoping to undermine the economic bases of Shiite activism. But Shiites who have lived and worked in the Eastern Province say that while material conditions have improved somewhat, grievances fester because of religious and political repression.

In a report issued last January, Amnesty International said most political activists arrested in Saudi Arabia and detained without trial were Shiites from the Eastern Province. These activists "are usually in solitary confinement, routinely tortured or ill-treated, and denied access to family and legal counsel."

Amnesty International documented the cases of dozens of reported Shiite detainees, including that of a housewife from the Eastern Province who was arrested at a border checkpoint for carrying a photograph of the late Iranian leader ayatollah Ruhollah Khomeini and then died in captivity, allegedly after three days of physical torture by Saudi guards.

Such reported abuses by Saudi security forces may well keep Shiites in the Eastern Province temporarily in check, as happened during the most volatile days of Iran's revolution. "We cannot do anything here or there," said one Shiite resident of Bahrain's slums. "If you try, you will be arrested."

Moreover, not all Shiites in Saudi Arabia look to Iran's radical leaders for guidance. Some were put off by the bloody purges that followed Khomeini's seizure of power, while others take inspiration from a Shiite ayatollah based in Iraq who advocates the separation of religion and politics, according to local Shiites and Western specialists.

Saudi Shiites also feel little affection for Iraq's Saddam, a Sunni secularist who at times has repressed his country's Shiite minority. Like Iran's leaders, Saudi radical Shiites may be content to watch a formerly distant enemy, the United States, neutralize a nearby one, Saddam, in the hope that Washington will then leave the Saudi peninsula to their own designs.