San Francisco Mayor London Breed had come to the Embarcadero to explain to a largely hostile crowd why she wanted to locate a Navigation Center for the homeless in the neighborhood. Just before she was shouted down, there was a telling moment.

Straining to be heard over cries of, “Build it somewhere else! Build it at City Hall!,” Breed replied, “I’m sorry this issue has divided this community.”

That may have been an understatement.

The proposal to build the city’s largest Navigation Center on a 2.3-acre parking lot across from Piers 30-32 has inspired a bitter backlash from waterfront residents who fear the plan will turn their neighborhood into a dirty, dangerous place.

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Their opposition has been met with raw antipathy across the city, including from other waterfront residents who see the center as a moral imperative. Addressing San Francisco’s homelessness crisis, they say, requires using any viable site to shelter people.

The issue has accentuated long-standing divisions within District Six, a piece of San Francisco that can be defined by its stark differences — a place where shimmering new condominiums and office towers cast long shadows over some of the city’s poorest and neediest residents.

“District Six is just lines on a map. This neighborhood has almost nothing to do with the Tenderloin,” said Matt Carson, who supports the Navigation Center, which would be built a block from his home in Rincon Hill.

The district is home to what is by far the city’s largest population of homeless people and the biggest concentration of homeless shelters and services. That’s become a major point of contention in the debate over the Embarcadero Navigation Center, and one that spurred District Six Supervisor Matt Haney‘s plan to introduce legislation this week to compel the other board members to open shelters and Navigation Centers in their districts over the next 30 months.

Many waterfront residents argue that since District Six has already shouldered more than its fair share of homeless shelters and services, city officials ought to build the center elsewhere. For that, they have been slapped with what’s become one of the most disparaging labels in San Francisco’s current political climate: NIMBY — short for “not in my backyard.”

“We’re not NIMBYs. We’re compassionate, but we’ve been the dumping ground,” said Robert Rossi, who opposes placing a Navigation Center on the parking lot at Seawall Lot 330 in his neighborhood.

That argument doesn’t sit well with Mike Sizemore, who lives just over a mile away from Seawall Lot 330 and kitty-corner from another District Six Navigation Center at Fifth and Bryant streets. Sizemore also wants to see homeless services spread across the city, but said the waterfront opponents were using the district’s boundaries to launder a revulsion at having a shelter so close to home.

“It frustrates me so much that they take ownership of fights that they would never fight,” he said. “This idea that they’re champions of homelessness services is just complete hypocrisy. Do you think any of these people are going to be in the middle of the Tenderloin, saying Glide (Memorial Church) should get more funding?

“If we’re actually concerned about homelessness, we should be talking about how we start, not stop projects.”

Sizemore credits the Navigation Center near his home with creating a “stabilizing presence” in the neighborhood.

“People say, ‘Oh, It’s going to bring crime and people loitering.’ I see in real time how well that thing is managed. If you’re concerned about safety, this is not the thing to be worried about,” he said.

At the Civic Center Hotel, a Navigation Center on 12th Street, a light breeze on Friday afternoon stirred up small cyclones of trash. Jason Woods, who works at a paratransit organization perhaps 100 yards from the shelter, agreed the street could be cleaner but said he’s never had a problem with the center’s residents.

“They don’t bother anybody,” he said. “They’re actually pretty nice. If you talk to them they’ll make conversation with you.”

Trei Johnson, who works at a general contracting firm next door to the center, said he’s never felt unsafe, but added that tents tend to mushroom on both sides of 12th Street, particularly on the weekends.

Travel 2½ miles east and the environment and attitudes change drastically. A community coalition called Safe Embarcadero has sprung up to protest the waterfront Navigation Center — their slogan is “stop the ‘mega-shelter.’” The group has raised more than $100,000 to fund legal action against the city if the center is built. A rival campaign to support the center has raised $175,000.

The opponents argue that the Embarcadero neighborhood, teeming with tourists, families and seniors, is ill-suited for a shelter. Many are also worried about public safety, and that the center’s homeless residents will bring crime, drug use and trash to the community.

Assurances from city officials, including the Police Department, that the center will be kept safe and clean meet pushback from residents’ stories of harrowing altercations with people they believed to be homeless.

Jeff Kositsky, executive director of the city’s Department of Homelessness and Supportive Housing, has said repeatedly that “Navigation Centers have generally made neighborhoods safer” and that crime and mental health issues shouldn’t be conflated with homelessness.

Many critics also say they felt ambushed by the mayor’s proposal and wished her administration had done more outreach before announcing a plan to build the center. Breed’s administration and Haney have conducted dozens of community meetings, large and small, over the past month.

“There are better places and there are worse places, and it’s hard to imagine a worse place to build (a Navigation Center) than on the Embarcadero, in a residential area,” said Wallace Lee, a waterfront resident affiliated with Safe Embarcadero.

“We’ve been called NIMBYs, or that we don’t care about the homeless. There have been all these moral judgments. I want to stress, it’s not about that,” Lee said. “This is a concern about crime, public safety and drug use. When you see the police drive by drug users in the city and not doing anything, you wonder, what’s the point?”

The vigorous opposition — including the threat of a legal fight to block the center — and Haney’s proposal also raise questions about future fights around homelessness services.

“We need to have a more equitable distribution of services and shelter and housing in this city,” said Haney, who supports the Embarcadero shelter even though his district already has five adult shelters and two Navigation Centers. The difficulty of finding viable spaces for shelters — officials turn down spaces for all kinds of reasons, including cost and safety concerns — make his plan an audacious one.

“If my friend, (District Two Supervisor) Catherine Stefani finds a site on Lombard, and she can go out there and say, ‘This is going to be put here, and I support it. But it’s part of a larger plan, and all of my colleagues on the board are a part of this’ ... I think that would help her, and make everyone feel like they weren’t targeted,” Haney said. “They’ll feel there was some reason behind it.”

Breed has set a goal of 1,000 new shelter beds by the end of 2020. So far, she’s opened 212. Homelessness officials estimate there are more than 4,000 people living on San Francisco streets; more than 1,100 are on the city’s shelter waiting list each night. One recent count on the waterfront found 179 people sleeping outdoors, according to Kositsky.

Breed wants the Embarcadero center built by this summer and plans to have it operate for four years. She and her staff say they’re willing to negotiate over the length of the lease and, to an extent, the size of the center. The lot is owned by the Port of San Francisco, which means Breed needs the Port Commission’s approval to lease the land and build the center.

Nearly two months of consternation over the center will come to a head on April 23 at San Francisco’s Ferry Building. That’s when the Port Commission, whose five members are appointed by the mayor, will vote on leasing the lot.

At least three commissioners appear poised to give the go-ahead: President Kimberly Brandon, Vice President Willie Adams and Commissioner Victor Makras. Makras said he’ll support the center, but wants the city to have alternative locations at the ready should the port receive a money-making proposal for the site, long the target for developers.

Breed has signaled her willingness to confront controversy and weather backlash over her policies on homelessness, which, she says, she was elected to curb.

“Building this Navigation Center ... is an important part of our work to address homelessness, which we know is the number one challenge facing our city,” she said in a statement. “If we are going to make a difference on homelessness in our city, we need solutions to help people off our streets and into housing and services.”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa