In a colorful ceremony designed to recall the opening of the Second Vatican Council, Pope John Paul II opened a world assembly of Roman Catholic bishops today to reassess the modernizing trends that the council set in motion 23 years ago.

A procession of more than 300 cardinals, bishops and priests, garbed in white-and-gold robes and white miters, streamed through St. Peter's Square to attend mass in St. Peter's Basilica. The rite was reminiscent of the 1962 opening ceremonies of Vatican II, presided over by Pope John XXIII.

John Paul officially inaugurated the Extraordinary Synod of Bishops, which will be in session until Dec. 8, during a brief sermon given before 10,000 delegates, dignitaries and faithful who jammed the basilica.

"It is a very significant fact that . . . we begin the synod at this eucharistic celebration with the same openness to what the Holy Spirit may say, the same love for the church, the same gratitude to divine providence that filled the council fathers 20 years ago," the pontiff said.

During the homily, he did not mention any specific issues facing the church. The extraordinary synod is a consultative meeting and cannot directly set policy for the world's 800 million Catholics.

Details of the synod agenda were not made public, nor was it known whether a formal document would be issued at its conclusion.

Vatican spokesman Joaquin Navarro said that the pope's opening remarks were general because he "did not want to push them in any way."

When he announced the synod last January, John Paul said it would have three purposes: to revive the "extraordinary atmosphere of ecclesial communion that characterized" Vatican II; to exchange information on the implementation of the council in various countries, and to "promote the further study and constant incorporation of Vatican II into the life of the church, in light of new exigencies as well."

That last phrase about new conditions has given rise to widespread speculation that the synod might roll back drastically what church liberals view as a spirit of freedom and openness that has characterized much of post-Vatican II Catholicism. But the pontiff appeared to lay to rest the persistent rumors that the synod would be the vehicle for reversing Vatican II teachings.

Msgr. Corrado Balducci, a Vatican official of the Congregation for the Evangelization of Faith, acknowledged, however, that while "the council gave the church a burst of optimism and a modern image," it also "created confusion and gave rise to certain exaggerations. So the synod will take another look at those reforms and, if needed, suggest to the pope a fine tuneup."

After a general report Monday, the 220 delegates and official observers will break into small groups for a week of discussions before returning to plenary sessions.

In addition, many of the 102 presidents of national bishops' conferences have submitted advance commentaries on the progress of Vatican II changes.

It is the first time representatives of other Christian communions are taking part in a synod; there are 10 observers from churches with which Roman Catholicism is in ecumenical dialogue.