The federal Commission of Fine Arts called on District of Columbia officials yesterday to impose an immediate moratorium on construction of enclosed sidewalk cafes, which city restaurants have built illegally and with increasing regularity during the last several years.

About a dozen of the more than 100 outdoor cafes built in Washington over the past decade are now enclosed, some in wooden, steel and plastic structures that federal officials say occasionally look like "sidewalk shacks." The structures have often taken up large portions of public sidewalks on some downtown Washington streets and along Connecticut and Wisconsin avenues NW.

A public hearing was held last week on a City Council bill to legalize enclosed sidewalk cafes and to impose regulations on them. Fine Arts Commission secretary Charles H. Atherton said yesterday that his agency, the 70-year-old watchdog on esthestics in the nation's capital, fears that many restaurants will rush to enclose outdoor cafes in hopes of being "grandfathered in," and thus not subject to any new regulation because they were in existence before a law could be passed.

While the Fine Arts Commission has no outright authority to order the city to ban further construction of the enclosed sidewalk cafes, its recommendations carry considerable weight and are almost always accepted by the city.

Chris Kefadas, owner of Maggie's Restaurant at 4237 Wisconsin Ave. NW, which is now enclosing its two-year-old outdoor cafe, said yesterday, "Everybody's doing it . . . and I just figure it will help increase business." He denied, however, that he was doing it in an attempt to evade the pending city regulations.

Council member David A. Clarke (D-Ward 1) said he introduced the Enclosed Sidewalk Cafe Act of 1981 because "nothing's being done by city officials to enforce the present regulations and there's been a proliferation of illegal" sidewalk cafes. Clarke said he strongly opposes any grandfather clause that would exempt existing sidewalk cafes from the proposed regulations.

The city has not attempted to block the enclosing of sidewalk cafes since it lost a 1977 court fight with the Paradise Cafe, now called Rumors, at 1900 M St. NW. "The court claimed the city's l961 outdoor cafe regulations were too vague to be enforced," Atherton said.

Most restaurant owners know their enclosed cafes are technically illegal, but expect them to be made legal soon and are reluctant to forgo the business that the sidewalk restaurants attract.

"I know it is against the law . . . and it's about time they made it legal," said Dennis Coughlin, general manager of Armand's Chicago Pizzeria at 4231 Wisconsin Ave. NW.

Clarke's bill would legalize enclosed sidewalk cafes for seven months of the year, from mid-October to mid-May, and at other times when temperatures are below 65 degrees or above 85 degrees. It also requires them to be removable within 24 hours and would double the annual rent, based on property values, paid to the city for outdoor cafes.

At last week's council hearing, the National Capital Planning Commission and the Committee of 100 on the Federal City, Washington's oldest planning body, urged that enclosed sidewalk cafes be limited to no more than 50 percent of the width of a sidewalk. That is similar to the 1971 New York City sidewalk cafe law upon which Clarke largely modeled his proposal.

Under present law, an outdoor cafe can be as close as 6 feet to a curb, which planning officials say leaves too little room for pedestrians.