Roman Catholic bishops from around the world began meeting today in an extraordinary synod to assess the impact of the Second Vatican Council, and two ranking cardinals said there is no turning back from that historic conclave.

Following a religious ceremony yesterday in St. Peter's Square led by the pontiff, 159 bishops today attended the first working meetings of the two-week conference, a morning session marked by two keynote speeches and an afternoon of debate at which 16 other bishops spoke. Pope John Paul II, who called the synod last January, was present at both sessions.

At a press conference after the morning session, the synod's chairman, Belgian Cardinal Godfried Danneels, said, "The council remains valid, completely valid. It is impossible to regress." Danneels had told the synod earlier about the responses of bishops to a Vatican questionnaire on the state of the church and the world's 800 million Roman Catholics that was sent out earlier this year.

Danneels was echoed by Cardinal John Krol of Philadelphia, who said that even though the liturgical, structural and social reforms engendered by the Second Vatican Council had caused "some turbulence, some dust," its "implementation . . . exceeded the great hopes of many members of the council."

Krol, one of the synod's co-presidents, also pointed out that the synod is strictly an advisory body and does not have the power to alter the landmark decisions of the 1962-1965 council. It could only seek to deepen understanding and perhaps provide better guidelines, he said.

In general, the two churchmen appeared eager to dispel images of the synod as a contest between Roman Catholic progressives like Bishop James Malone, who heads the U.S. Episcopal Conference, and conservatives like Cardinal Joseph Ratzinger, head of the Holy See's Congregation for the Doctrine of the Faith.

"It is not a boxing match or a conflict," Krol told more than 500 reporters gathered this afternoon at the Vatican press center.

Observers at the synod's afternoon session, the first in a scheduled four days of general debate, said the first 16 speakers, including bishops from Latin America, Canada, South Africa, Africa and Asia, expressed similar views.

"There was a general consensus that the Vatican Council represented a moment of grace in the life of the church," said the Rev. Diarmuid Martin, a Vatican spokesman.

The cardinals made no attempt today to hide the fact that different opinions exist about the implementation of the council's decisions as well as its current relevance. "There is unity but not uniformity within the church," Krol said.

Danneels pointed out that some of the major issues for the church in the western world, like the role of women within the church, or collegiality -- the relationship between the pope and his bishops -- are not top priorities in Africa or parts of Asia or Latin America.

Danneels noted that their different viewpoints could lead some of the synod's participants to say, looking at the same glass of water -- that is, the council -- that it was half full or half empty. But, he said, "what we're really interested in" is "how much water there is in the glass."

The two prelates stressed that there will be complete freedom of expression at the meeting. In his opening remarks yesterday, the pope had urged frank and open discussions.