By Dave Warner

PHILADELPHIA (Reuters) - One of the most closely watched sex abuse trials involving the Roman Catholic church began on Monday and a prosecutor asserted that Monsignor William Lynn was the "keeper of secrets" who failed to protect children and then covered-up crimes.

Lynn, 61, the most senior cleric to go to trial in a wave of sexual abuse cases against the Catholic Church in the United States, is charged with endangering the welfare of children and conspiracy, for covering up allegations against priests.

Lynn served as secretary of the clergy under the late Cardinal Anthony Bevilacqua, Philadelphia's archbishop from 1988 to 2003. That made Lynn, in effect, personnel director for priests in the diocese.

"The defendant is the keeper of secrets," Assistant District Attorney Jacqueline Coelho told a Philadelphia jury in her opening statement on Monday. "The protection of children is the furthest thing from defendant Lynn's mind."

Instead, she said, his chief concern was "to keep a lid on scandal."

Bevilacqua died in January at age 88, but his videotaped testimony could be introduced during the trial.

The Vatican is closely watching the criminal trial involving the sixth-largest diocese in the United States, said Terry McKiernan, co-director of BishopAccountability.org, a watchdog group.

"This is cause for major anxiety in the church," McKiernan said.

Four others were indicted with Lynn, including Reverand James Brennan, 49, who is charged with child abuse and is on trial with Lynn. Defrocked priest Edward Avery, who will also go on trial with Lynn, pleaded guilty last week to sexually assaulting a 10-year-old altar boy in church in 1999.

Another priest and a former archdiocese school teacher will be tried at a later date.

Lynn was indicted by a Grand Jury in January 2011.

"Over the past two decades, Msgr. Lynn has put literally thousands of children at risk of sexual abuse by placing them in the care of known child molesters," the Grand Jury said in its report that year.

"We believe that legal accountability for Msgr. Lynn's unconscionable behavior is long overdue, and that he should be prosecuted for endangering the welfare of the victims in these cases."

Other criminal cases against senior Church officials in the United States have ended in plea bargains, effectively eliminating the chance for probing questions to be asked in open court.

The cost of the scandal - particularly from settling a flood of victim lawsuits - has already bankrupted eight dioceses, most recently the Archdiocese of Milwaukee as well as the diocese of Wilmington, Delaware, which has sought Chapter 11 protection.

Lawsuits against the Philadelphia Archdiocese have been filed on behalf of six alleged victims of sex abuse by priests, but those cases have not yet gone to trial.

Perhaps the most high profile case to date occurred in 2002 in Boston, where hundreds of people said they had been molested by priests and ultimately Cardinal Bernard Law lost his job as the head of the archdiocese.

(Editing By Barbara Goldberg and Paul Thomasch)