NEW YORK — The priest looked out Sunday afternoon on a rare sight: Every pew in his church was packed, the crowd so thick that some had to stand.

They had come to show support for his church, Our Lady of Peace on the Upper East Side, one of 112 parishes the Archdiocese of New York plans to close or merge as church attendance wanes and finances weaken. The sizable attendance was a moment that called for thanks, and perhaps, the priest said, for acceptance.

“We hope that your presence here today will strengthen us and make it possible for us to continue as Our Lady of Peace,” the Rev. Bartholomew Daly said, standing beneath the church’s ceiling of painted saints and Venetian glass chandeliers, imported from Italy after the church’s founding in 1918. But, he added: “We dread any kind of change, but maybe God has something marvelous for us, something we cannot see now. God is a God of surprises, and he gives us what we don’t always expect.”

For now, though, his flock is determined to fight the changes the archdiocese is sending its way.

Since Our Lady of Peace was slated for closing last year, parishioners have orchestrated a dogged campaign to save it, despite the archdiocese’s declaration that its decision is final. At the 12:30 Mass on Sunday, they packed the pews with friends, former parishioners and others as part of a “Mass mob,” a flash-mob-like gathering designed to bring attention – and donations – to individual churches.

Mass mobs have sprouted around the country, drawing crowds to struggling churches. In New York, where churches are selected for Mass mobs based on online “elections,” Our Lady of Peace was the first church to benefit from such a gathering.

On Sunday, donation baskets were piled with $20 bills. Children in the doorway handed visitors rubber bracelets printed with the church’s name. More than 300 people came, some inspired by social media, some in protest of the archdiocese. But most were parishioners or former parishioners who could not bear the thought of the closing of the church where they had celebrated baptisms, first communions and weddings.

“What the archdiocese wants to see is more vibrant parishes,” said one parish leader, Janice Lynch, after the Mass attendees had gone into the church’s basement for Italian cookies, coffee, and rugelach. “How can you be a more vibrant parish than this one?”

Rama Zomaletho, 31, does not belong to the parish, but his infant daughter was baptized there a few months ago. The parish and Daly were so welcoming, Zomaletho said, that he and his wife have decided to leave their Bronx parish and join Our Lady of Peace — provided that it stays open.

“It would be a shame to close this,” he said. “It’s beautiful, just to look at it.”

Their parish in the Bronx, Our Lady of Grace, was also scheduled to close, but Zomaletho said he hoped that stronger attendance and repeated protests there would push the archdiocese to reverse its decision. Parishioners say the reasons they are fighting for Our Lady of Peace go beyond aesthetics, though the building, which was built in 1866, is part of a historic district. They point out that the church is financially self-sustaining. Its attendance is healthy, they say, and it plays an important part in community programs like Alcoholics Anonymous.

It also serves as a sanctuary to Catholic workers at Bloomingdale’s and other nearby businesses.

“This, in my heart, this is my parish,” said Dee Johnson, 63, who used to come to Mass on her lunch break from Bloomingdale’s every weekday and still returns when she can, 14 years after retiring.

Like Johnson, Silmara Roman, 40, who was married at Our Lady of Peace in 2000, made a special effort to go to the Mass on Sunday, though it is not her regular time for visiting the parish she left six years ago. She had even skipped a family gathering hosted by her mother-in-law to make the trip from Queens, where she lives now.

“Having an Italian mother-in-law, it’s hard to say no. But I told her this will make me more Italian, coming to Mass,” she said, laughing. “So many people came back today. Oh, my God, it was awesome to see.”