Vandals did extensive damage late last week to the Islamic Education Center in Potomac, a combination school/social center and political headquarters for Iranians who support the regime of revolutionary leader Ayatollah Ruhollah Khomeini.

It was the second such assault on the center in the last two months -- since June 14, when Shiite Moslem radicals hijacked a TWA jet to Beirut. This time, the vandals, whom police have not been able to identify, smashed windows on three sides of the building, shattered all windows and headlamps of a Volvo station wagon parked nearby, and spray-painted garish red and black graffiti inside the center and out.

Shiite residents of the Washington area say such petty violence is nothing new.

*Georgetown rug merchant Bahram Nahidian, one of the most prominent pro-Khomeini activists at the IEC, said that one of his cousins who "wears traditional dress" returned home only a few days ago to find his apartment windows broken and the apartment littered with broken eggs.

*The day the story of the hijacked jet broke, a Lebanese-American student recalled recently, she was working parttime in a Washington boutique. "I was over in the corner folding clothes and the other girls were reading the newspaper," she said, "and they were saying things about Shiites, saying there were too many 'i's' in it, and one of them said if she had a gun she'd shoot every last Shiite . . .and I acted as if I were just as angry. I was ashamed to tell them I am Shiite. I was afraid."

*A Montgomery County Shiite businessman said that when his 8-year-old son came home from school during the hijacking asking, "Daddy, what religion are we?" he answered only, "We are Moslem."

"I figured that by the time he discovered that it's the same thing, he would be old enough to understand and to deal with prejudice intelligently," the father explained later.

The distinction between Shiites and other Moslems is very fine -- "in fact, my husband didn't even know there was more than one kind," according to Lebanese-born interior designer Salwa Saleh. Shiite and Sunni (mainstream) Moslems are separated primarily by their allegiance to rival successors to Mohammad.

There is little difference in religious practices; the sects worship jointly at the area's only mosque on Massachusetts Avenue in Northwest Washington, and several area residents repeated that if asked their religion, they respond only Moslem.

"They would say 'Moslem' in the same way Protestants would say 'Christian,' " said Jim Zogby, head of the Arab-American Institute.

Armchair political analysts might be surprised by the number of Shiites living here. Nearly all the estimated 30,000 to 40,000 Iranians in the metropolitan area are Shiite, though they are bitterly divided by their support or contempt for Khomeini. A small percentage of the 12,000 Arabs here is Shiite -- "handfuls," one suggested -- along with students and transient businessmen. There are also a few Shiite immigrants from such countries as Pakistan, India, Ethiopia and Afghanistan. (In addition, there are perhaps 300 Druze, a conservative offshoot of Shiism.)

Critics of Khomeini blame the media label "Shiite terrorist" on Khomeini's glossing his reactionary regime as Shiite, and to his efforts to incite "religious" revolution in other Middle Eastern countries.

"As Shiites, we do not consider Khomeini Moslem," said Ali Safavi, former anti-shah activist and now spokesman for the local supporters of the national anti-Khomeini Mujaheddin movement. "The message of Khomeini's Islam seems to be one of ignorance and terror, one of backwardness . . .but this is in direct conflict with what Islam teaches."

Pro- and anti-Khomeini Iranians in the United States feed local and federal law enforcement officials a steady stream of rumors about each other. In the Washington area, for example, the fundamentalist followers of Khomeini and the comparatively leftist members of the Mujaheddin wage a constant war of pamphlets and posters, fingering pro-Khomeini students as "subversive agents" and anti-Khomeini students as "decadent" and "corrupt."

Many Shiites in Washington are comparatively liberalized, wearing Western dress and indulging in the traditionally prohibited pleasures of tobacco and liquor. "As you see, I am one of the 'Westernized,' " said a businessman, lifting his drink. ("Who would return to the Middle Ages?" shrugged another.) Safavi's group is a mixture of political progressivism and cultural protectionism: He wears pinstripes but abstains from alcohol, and women among the Mujaheddin wear scarves in place of the all-concealing chador.

The approximately 200 Iranian followers of Mohammad Asi, the local "imam" at the Islamic Education Center in Potomac, are extremely conservative; men as well as women cover their heads. They were expelled from the mosque following an abortive fundamentalist takeover, and their secretiveness and separatism is so intense that only a few outsiders, including other Iranians, are aware of the center's existence.

Ironically, considering the traditionally working-class and rural status of Shiites in the Middle East, Shiites in the Washington area point to themselves as disproportionately wealthy and well-educated. Some say they are here precisely because of the educational opportunities, and often they represent the first literate generation in their families.

"Education is an obsession" among the Shiites, according to the lawyer. "I know a woman who used to carry milk on her head to sell. Now one son is a doctor and one is an engineer."

"They work for the money to send their children to study here," he continued. "Those who pass through the bottleneck, they are the lucky ones. Most of them will say they came not to immigrate, but to study, but that is a kind of nostalgia. Most will stay here."

In contrast, the only organized Shiite community in the United States, in the Detroit suburb of Dearborn, was founded in the 1960s by poor immigrants seeking factory work. It now includes an estimated 10,000 people, and Washington Shiites joke that Dearborn stores have signs saying, "English spoken here."

As observers point out, Shiites, traditionally in the minority in Arab countries, had less to lose by emigrating than other Arabs. Shadab notes that a high percentage of Iranian immigrants are doctors, engineers, lawyers and former government employes who have fled the Khomeini government.

Among some emigrants, the petty violence they've experienced in this country and the media's regular use of the word "Shiite terrorist" has sparked a certain bellicosity on their part. "It is the media who distort such things," said rug merchant Nahidian. "You want to call [someone] a terrorist, that's your problem. We'd call him a defender of the land."

But stereotyping is not confined to this country, said Hazem Baghdadi, a former journalist now at the World Bank. "The last time I was visiting with one of my cousins -- and he and I were very, very close growing up -- I sensed a coolness toward me. It was as though now that I'm an 'American' I can't understand them anymore."