The Roman Catholic Archdiocese of Boston on Thursday announced the names of more than two dozen parishes participating in the first phase of a major reorganization that will eventually group the archdiocese’s 288 parishes into about 135 clusters, each led by a team of clergy and lay leaders.

The reorganization, to be phased in over five years, is designed to help parishes cope with diminished Mass attendance, a shortage of priests, and anemic fund-raising. Church officials hope the plan will eventually strengthen parishes and help reverse those trends.

The 28 parishes participating in the pilot phase—diverse in size, wealth, ethnicity, and geography—will be grouped into 12 clusters, or “collaboratives.” The collaboratives will gradually take shape over the next two years, as clergy and lay leaders are assigned and trained, and teams from each one create a long-term plan.

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The parishes will remain open, but church officials said they hope will learn to work together to share resources.

The Rev. Paul Soper, director of the archdiocese’s Office of Pastoral Planning, acknowledged that the process will be protracted, complex, and somewhat bureaucratic, but he said the plan is ambitious, and the archdiocese intends to take the time necessary to make the plan work.

He added that many of the priests he has spoken with see this as a “rational, moderate plan” that keeps parishes open but tries to set them on a more solid course for the future.

The challenge, he said, will be “staying open to the movement of the Holy Spirit as we move through this, while at the same time as being attentive as we can to every detail.”

He said most of the parishes participating in the first round volunteered—their priests consulted with parish councils or the congregation and stepped forward. The rest agreed after some discussion.

After encountering vitriolic resistance to forced church closings in 2004 and 2005—the church’s last effort to downsize—the archdiocese has taken care to develop this new plan slowly, after broad consultation with parishioners and clergy, and to present it as an effort that will unfold naturally over time.

Soper said the first round of collaboratives was chosen to help test the reorganization plan in different settings, with varying logistical and cultural complexity.

One of the collaboratives contains only one parish: St. Mary of the Assumption in Brookline, a financially sound, English-speaking parish that is the only Catholic church in town.

The Salem collaborative, on the other hand, includes four parishes that hold Masses in three languages—English, Spanish, and Polish—and serve many low-income households.

Each collaborative will be assigned a lead pastor by April, Soper said. Pastors will undergo training in May and June, move if they are coming from outside the collaborative by the beginning of the summer, and then begin assembling their teams of assistant clergy and lay councils.

Soper said the archdiocese had not yet determined how many priests would be assigned to each collaborative.

He acknowledged that the transition period may leave some parishioners on edge but said it was necessary to allow enough time for the pastoral selection process, overseen by the archdiocese’s clergy personnel board and, ultimately, Cardinal Sean P. O’Malley.

The pastoral assignments for the collaboratives “are the most important choices we are making,” Soper said. “It’s much better to make those choices carefully than to make them quickly.”

All the staff and lay leaders will be in place by fall, when the archdiocese will provide them with 16 days of training over several months.

Then, a team of people in each collaborative will spend a year developing a long-term plan outlining how they will attend to their parishioners’ needs and how they will expand efforts to evangelize, working to bring more people to church.

Today, fewer than 20 percent of baptized Catholics in the archdiocese attend Mass, down from 70 percent several decades ago.

The archdiocese hopes that by using resources more efficiently, the parishes will become stronger and more lively, which will help them grow again.

“The biggest challenge is always evangelization—being out in front and keeping people focused that this is about our love relationship with the Lord,” Soper said.