Archbishop James A. Hickey was formally installed yesterday as the leader of the Washington archdiocese's 400,000 Roman Catholics and in a solemn ceremony at St. Matthew's Cathedral pledged to serve "all who live in this region."

"I am anxious to listen to you, to your concerns, and extend my hand in partnership to all who seek to nourish and enrich the family of man," Hickey said in his homily.

He expressed special greetings for "dear sisters and brothers of the Jewish faith . . . my sisters and brothers who profess the faith of Islam" and Christians of other traditions.

Addressing non-Catholic Christians, he said "our fervor and efforts for Christian unity must be increased . . . I rededicate myself to every level of ecumenical activity with a renewed hope and in complete honesty."

More than a score of non-catholic religious leaders, including yellow-robed Buddhist monk, marched in the ecclesiastical procession.

The attendance by non-Catholic religious leaders was not new. What was unique, however, was Hickey's warm and expressive style -- a sharp contrast with the more formal bearing of his predecessor, Cardinal William W. Baum.

As each of a score of greeters chosen to represent the varied constituencies of the archdiocese approached Hickey during the service, for example, he enveloped several in bear hugs and held a brief conversation with each.

An especially moving moment came at the beginning of Hickey's homily, only moments after Archbishop Jean Jadot, official representative of the Vatican in this country, had escorted him to the marble-and-gold archbishop's chair and handed him the long, gleaming shepherd's crook, the symbol of his office.

"What does a father say on being united with his family?" Hickey began. "What does a brother say to newly met sisters and brothers? Only this, I love you. In this grace-filled moment I stand in the presence of the church of Washington . . . We are now one family in the Lord."

Hickey singled out for special recognition a number of individual groups within the archdiocese, including black and Hispanic Catholics, the priests, the religious orders in the area and the various institutions of Catholic education.

He told Catholics not to be concerned only about "ourselves and our personal lives alone . . . In our day the poor, the handicapped, the victims of racial, ethnic and religious hatred, those alienated from the church and the unchurched must be a cause for our special concern."

Civic leaders attending the invitation-only ceremony included Mayor Marion Barry and Maryland's Lt. Gov. Samuel W. Bogley Jr. Cardinal Baum also took part.

Hickey, 59, is the former bishop of Cleveland. He will serve as the fourth archbishop of the Archdiocese of Washington, which includes 128 parishes in the District of Columbia and five Maryland counties -- Montgomery, Prince George's, Calvert, Charles and St. Mary's.