The head of American Roman Catholic bishops yesterday praised and defended the modernizing changes of the Second Vatican Council and predicted that the worldwide Synod of Bishops, meeting in Rome later this month, will reaffirm them.

The teaching of Vatican II, which ended 20 years ago, "has been received in the United States as a blessing; the implementation of the Council has been fundamentally sound and faithful," Bishop James W. Malone of Youngstown, Ohio, told the National Conference of Catholic Bishops at its annual session here.

Malone, who as president of the organization, will represent the American church at the synod, said, "I find dismaying the voices which speak of the synod with apprehension. I see the synod as an opportunity to enhance, broaden and deepen the process of faithful implementation of Vatican II."

Among the changes in the church that came out of Vatican II, Malone cited "startling progress" in relations with other churches and the development of a strong voice on questions of social policy, "ranging from issues of medical technology to issues of nuclear technology."

Archbishop Pio Laghi, the pope's personal representative to this country, yesterday praised the American bishops for their 1983 pastoral letter condemning nuclear war and for the pastoral they are working on about economics.

The praise of the papal representative is significant in light of widely publicized criticism from a powerful Vatican official, Cardinal Joseph Ratzinger, that bishops' conferences have too much power.

Laghi, in his annual address to the bishops, said the pastoral letters of the American bishops and the "collegial," or cooperative, style of developing them, "are important examples of how a conference of bishops functions today in offering . . . leadership on public issues."

The development of collegial relationships between bishops and the pope and among bishops themselves has been one of the most significant -- and controversial -- developments in the church, which for nearly 2,000 years has been structured around a hierarchy.

Liberals fear and conservatives hope that the Synod of Bishops, which begins Nov. 24, will curtail some Vatican II reforms, including collegiality.

Malone, who traced the development of a strong conference of bishops to the post-Vatican II collegial principle, said collegiality will be "a major topic" at the synod.

Most of today's sessions were taken up by the introduction of proposals the bishops will act on later in the week. Included was a new antiabortion program of political action for the passage of a constitutional amendment "providing protection for the unborn child to the maximum degree possible."