Washington-area Catholics turned out by the thousands yesterday to look over "the new baby," as Archbishop James A. Hickey described himself, at a mass and family festival at the Shrine of the Immaculate Conception.

Although Hickey formally was installed as head of the 400,000-member archdiocese last week, yesterday's mass and celebration marked the first chance for most church members to meet their new leader. An estimated 8,000 persons jammed the aisles and passageways of the Shrine to pray with the archbishop, snap his picture and later, at the festival, to shake his hand. Thousands more waited outside for the celebration to begin.

In his homily Hickey repeatedly told his new family, "I love you." The congregation responded several times by breaking the solmenity of the mass with applause. At the end of the homily, Hickey was given a long standing ovation.

Hickey had a special word for Catholics, "who feel estranged and embittered and isolated," including those who may be alienated from the church by divorce and forbidden remarriage. "To all of you I want to say, 'You belong to the church, we love you, we want to help in every way we can,' he said. "Perhaps the alienation can be readily resolved, but even if it cannot, be assured that I want to welcome you and work with you in Christ's name."

As he had done at this installation mass, the new archbishop, who came here from Cleveland, extended special greetings to the various ethnic groups in the archdiocese. The ethnic diversity was reflected in the prayers offered by laymen and women in seven languages, and in the entertainment at the festival -- the first of its kind -- that followed the service.

Hundreds of persons came in native costume: buckskin clad Indians, Vietnamese girls in creamy white bao dais, Korean women in diaphanous dresses and Latin-American women in brillantly embroidered dresses and shirts.

There were clowns, ballons, placards, political campaign-style straw hats all with the same message: "Welcome Archbishop Hickey." Throughout the festival Hickey worked the crowds, shaking hands, getting acquainted with his new family.