The Most Rev. James A. Hickey became the fourth Roman Catholic archbishiop of Washington last night during a vesper service at St. Matthew's Cathedral that was marked by bursts of prolonged applause from a gathering of archdiocesan priests.

Although Hickey will be installed formally at rites this afternoon, he officially assumed leadership of the archdiocese at the service when he handed his letters of appointment from the Vatican to Auxiliary Bishop Thomas Lyons.

After presentation of his credentials, Hickey embraced each of the five consultors of the archdiocese -- Auxiliary Bishops Eugene Marino, Thomas C. Kelly and Lyons, who has acted as administrator of the archdiocese for the past six months; Msgr. John F. Donoghue, the chancellor; and the Rev. John Scanlon, head of the senate of priests.

Then the 59-year-old former Bishop of Cleveland walked across the chancel to embrace with the man he is succeeding, Cardinal William W. Baum, and retired Cardinal Partick O'Boyle.

Earlier yesterday, Hickey pledged at his first news conference here to steer clear of partisan politics during his tenure, but at the same time to "speak forthrightly in matters that concern religious values."

His first priority, he said, would be the archdiocese's 128 parishes, which he said he intended to visit and consult with before announcing any new programs for the church here. "I have not come with all kinds of new programs in my briefcase," he said, "but I want to listen to my brother priests . . . to hear what their needs are, their proposals."

He added that it would "prejudge" this listening process "if I came up with specific answers to problems" of the archdiocese beforehand.

The archbishop refused to label himself as either liberal or consertative on theological issues. As archbishop of the Washington, he also becomes chancellor of Catholic University, where theologians have at times been at odds with the hierarchy. Ashed about this area of conflict in the church, Hickey said he is "very respectful of theology and theologians. . . . My whole life pattern has been one of collaboration with theologians."

While upholding the right of theologians to speculate and develop new approaches to church doctrine, Hickey added that "they need to test their conclusions" against the scriptures and tradition "the same way the scientific community" evaluates discoveries.

The new archbishop fielded questions with ease and laughed and joked with reporters in a relaxed manner. Afterward, he ignored aides waiting to usher his out, pausing instead to shake hands and chat with his visitors.

As a part of the installation week celebrations, Hickey is inviting families throughout the archdiocese to aget-acquainted fiesta Sunday afternoon, following a 4 p.m. mass at the Shrine of the Immaculate Conception. The festival is being held instead of the invitation-only formal banquet that has marked the installation of new bishops in the past.

Hickey asked that the festival be planned to provide a relaxed atmosphere and celebrate the cultural, ethnic and racial diversity of the archdiocese.

Many Washington Catholics see theirnew leader as uniquely suited to lead the diverse chuch here.

The Rev. Sean O'Malley, head of the Spanish secretariat, noted that as bishop of Cleveland, Hickey dispatcheda number of his priests to serve in El Salvador. According to O'Malley. "Many of the Salvadorans now in Washington have come from parishes served by these (Cleveland) priests."

Many of these people are anxious to greet the man who, indirectly, they viewed as their bishop, he said, "That's a very important plus," O'Malley added. Hickey was also one of U.S. bishops who attended the funeral of slain El Salvador Archbishop Oscar Romero.

As a priest of the Diocese of Saginaw (Mich.) where he later served as bishop, Hickey organized the Mexican Apostolate for Spanish-speaking migrants who follow Michigan's crop harvests. Later he established an office for Hispanic affairs in the Cleveland diocese.

Washington's black Catholics point to Hickey's forthright stands in Cleveland against racial segregation, particularly in support of desegregation of the public schools. Despite sizableopposition, he worked to ensure that church schools did not become an escape from desegragated public schools.

Catholic feminist hope the new archbishop will continue to appoint women to key church posts, as he did in Cleveland. In the Cleveland chancery, women's ministries. The latter post was established "to develop more jobs for wormen," a former aide to Hickey said.