Sunset is approaching. The mosque is ready for worship, its minaret a white pencil against the darkening sky. In ones and twos, men slip through the gate, pass under the Arabic arches of the colonnade, and go silently inside. It is time for evening prayer.

The scene might well be Riyadh or Islamabad - but it is Washington, D.C. The mosque is the US Islamic Center, a building that stands out among its Embassy Row neighbors like a man wearing desert robes in a crowd of three-piece suits.

Most Americans probably think of Islam as limited to North Africa and the Middle East. In fact, it is one of the fastest-growing religions in the United States. There are now more US Muslims than there are native American Indians, or Japanese-Americans, according to religious scholars.

They range from Lebanese PhDs to Yemeni cab drivers, Saudi students, and native-born blacks. Some are fundamentalists who reject Western values. Most are struggling to be Islamic and American at the same time.

''Islam is not only for Muslim countries,'' insists Muzammil Siddiqi, imam and director of the Islamic Society of Orange County, Calif.

Generally, Islam today draws attention in this country only when it is associated with dark and spectacular acts: coups, wars, assassinations. Events in the United States itself have at times served to reinforce the stereotype of Islam as a violent religion.

* In 1977, a band of black Hanafi Muslims - a tiny splinter sect - stormed three buildings in Washington, killing one person and holding scores hostage.

* In 1980, Ali Akbar Tabatabai, an Iranian exile and prominent critic of the Khomeini regime, was shot to death in the door of his Bethesda, Md., home.

But these events are aberrations, US Muslims say. Islam, they say, has had a rich and peaceful history in the US. Muslims helped settle many Midwestern industrial communities, they point out. At one time Muslims controlled the New York biscuit industry. They also participated in a short-lived attempt to raise pack camels for the US Army.

In the early 19th century, US intellectuals took a great interest in Islam as an exotic Eastern religion. ''You had that in Emerson, you had that in Thoreau, you had it in Melville, you had it in all these important thinkers,'' says Dr. Sulayman Nyang of Howard University.

In the 1890s, Alexander Webb, a US diplomat stationed in the Philippines, embraced Islam, moved to New York, and tried to spread the religion in America. But it wasn't until after the turn of the century that Islam took root in the US , as Muslims from Turkey, Lebanon, Syria, and Eastern Europe came here.

Many moved to the Midwest, where they became traveling peddlers or worked as laborers for the booming factories of the region. The mosque that was opened in Cedar Rapids, Iowa, in 1934, was perhaps the first mosque in the country.

The largest concentration of US Muslims is still in the working-class suburbs of Detroit, near automobile plants.

After World War II, Muslim professionals from India and other countries began migrating to America. Today, between 2 million and 3 million Muslims live in the US, according to most estimates.

Shia Muslims of Iranian heritage are the largest subgroup of the religion in this country, says Arif Ghayer, a sociology professor at St. Lawrence University. Syrians and Turks are the second- and third-largest subgroups.

Diplomats and foreign students attending US schools make up about 15 percent of the Muslims in the US; the rest are citizens or permanent residents.

Islam is not a major religious force in this country. (There are some 13 million US Methodists, for instance.) But Islam is growing rapidly here, because of loosened immigration restrictions and a relatively high birthrate among Muslims. Arif Ghayer estimates that the US Islamic population has increased tenfold since 1965.

''Muslims are perhaps the fastest-growing religious-ethnic group in the United States,'' he claims.

With this growth is coming some difference in character. Today, there is a new sense of self-awareness and self-assertion among many US Muslims.

For most this means becoming scrupulously religious for the first time: praying five times a day and studying the Koran. For a few it means political activism against what they perceive as un-Islamic regimes at home.

For many second-and third-generation Muslims, integration into US society has meant remaining circumspect about their religion. ''Those of us who have assimilated here - we did it in a different kind of way,'' says Sam Hamod, director of the Washington Islamic Center. ''We didn't always show our religion outwardly to people, but we practiced it very strongly.''

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