Pittsburg ? Joan Houk has ministered to the sick and needy, run two Roman Catholic parishes that were without priests and presided over baptisms and funerals. Her calling now, she says, is to be a priest.

Houk will be one of a dozen women participating in a ceremony today in which eight will proclaim themselves priests and four deacons. The ceremony won’t be recognized by the Catholic Church, which has a 2,000-year tradition of an all-male priesthood.

Similar ceremonies conducted by the group Roman Catholic Womenpriests have been done before in other countries, and most of the participants have been excommunicated. It’s the first time the group is having a ceremony in the U.S.

The Pittsburgh Diocese issued a statement saying the ordination would not be valid.

“This unfortunate ceremony will take place outside the Church and undermines the unity of the Church. Those attempting to confer Holy Orders have, by their own actions, removed themselves from the Church, as have those who present themselves for such an invalid ritual,” according to the statement released by the Rev. Ronald Lengwin, a spokesman for the diocese.

The diocese said it will welcome back anyone who chooses to leave the church.

Liberal Catholics say the ongoing clergy shortage and the dramatic rise in female lay leaders in American churches eventually will create pressure to ordain women. More lay people than priests are working full-time in American parishes and a significant number of the lay leaders are women.

Joan Houk, a Roman Catholic, looks up references to the priesthood in a reference book in the dining room of her Wexford, Pa., home. Houk will be one of a dozen women participating in a ceremony today in which eight will proclaim themselves priests and four deacons.

But conservatives believe only males can be priests, as evidenced by Jesus’ choice of men to be his apostles and the church’s long tradition of only allowing men to serve.

A majority of Catholic respondents to an Associated Press-Ipsos poll taken just after the death of Pope John Paul II last year said they favored ordination of women.

Womenpriests said a male priest presided over the first ordination of seven women sponsored by the group in 2002 in Austria; the women were excommunicated by the Vatican in 2003.

“We need to claim for women their equal right with men to be ordained. And we need to do this ‘contra legem,’ to break an unjust law and yet to remain firmly within the church,” Patricia Fresen said last year at a Philadelphia conference on women in the church.

She is a member of the group, in which she carries the title of bishop, and is one of three people presiding over the Pittsburgh ceremony.

They also say there are many examples of women with prominent roles in Jesus’ ministry, including his mother Mary and Mary Magdalene.

“Women were definitely leaders in the early church and so that’s what we’re asking to go back to, the early leaders of women in the church and leading to ordination,” said Aisha S. Taylor, executive director of the Fairfax, Va.-based Women’s Ordination Conference.