Residents of Rincon Hill, South Beach, South Park and Mission Bay jammed Tuesday’s Port Commission meeting in overwhelming opposition to Mayor London Breed’s plan to build the city’s largest Navigation Center on the Embarcadero just south of the Bay Bridge.

The prospect of bringing a 225-bed Navigation Center to Seawall Lot 330 inspired more than two hours of often-impassioned public comment from dozens of San Francisco residents, who outlined a long list of concerns.

And that was just the first community event about the center. A crowd of more than 200 people filed into a second meeting, organized by the Department of Homelessness and Supportive Housing, at the Delancey Street Foundation. Many speakers attended both meetings.

The 2.3-acre lot, directly across from Piers 30-32, is owned by the Port of San Francisco, so the city needs the commission’s sign-off to build the center. A commission vote is expected on April 23, following more community outreach by the homelessness department and other agencies. Breed wants the center open by this summer.

Under the mayor’s plan, the Navigation Center would operate for four years, though Jeff Kositsky, director of the homelessness department, said the city is still working out its final plan for the center, including the initial length of its lease.

He added that feedback from the community was central to that process.

“We’re in the beginning of a process of how to make this work best for your neighborhood,” Kositsky said. “The only way to provide people with assistance is to get them into a place of safety.”

While many of those opposed to the idea said they were sympathetic to the plight of the homeless, residents insisted the center would create a dirty, dangerous blotch on the housing-dense, tourist-heavy area.

They expressed deep skepticism about the city’s ability to effectively administer the intensive services that Navigation Centers offer. And they complained that they’d been blindsided by a proposal that felt imminent despite being introduced to the public just eight days earlier.

Several also recounted sometimes violent altercations with homeless people — a harbinger, they warned, of the kind of behavior the center would attract.

The Navigation Center would act as a magnet for “all the things normally associated with homelessness: criminal activity, drug usage and environmental pollution, both the visual kind and the physical kind,” South Beach resident William Glasgow said at the port meeting.

He wasn’t the only speaker who implored the city to weigh the benefits of a Navigation Center against possible detrimental neighborhood impacts.

“This will be a disaster,” attorney Robert Arns said at the port meeting. “I ask you this question: Will you take legal responsibility for the ramifications that will occur from this?”

“We’re all for helping the homeless find a better life. It has to be solved with dignity and respect. But it cannot be dumped on us, either,” Marcus da Cunha said in an interview before the port meeting. “We’re all for helping people, but why can’t we find another neighborhood to host this Navigation Center?”

The Navigation Center’s supporters were outnumbered at both meetings, but their message was clear: Finding shelter for the homeless was a moral imperative and one that trumps potential quality-of-life concerns.

“Every neighborhood says, ‘not our neighborhood,’” Kelley Cutler, an organizer with the San Francisco Coalition on Homelessness, said at the port meeting. “But the reality is, for the health of the community as a whole, we need to be creating resources and actually helping people.”

The center’s supporters occasionally had to speak over jeers at the Delancey event.

“The people who are opposed to this are making me deeply, deeply sad,” one woman told the crowd. Opposing the center means “you’re continuing to let elderly people die on the streets,” she said.

Bobak Esfandiari, a member of the San Francisco YIMBY (Yes In My Back Yard) organization, emphasized the city’s need to build more housing — and homelessness shelters.

“I want one of these in my neighborhood!” he said.

After public comment had ended and many of the port meeting’s attendees had moved on to the Delancey Street event, Kositsky countered that “homeless people are much more likely to be the victims of crime than the perpetrators of crime,” and that a study had shown property values have not decreased around existing Navigation Centers, nor has crime increased.

The new Navigation Center would be located in District Six, represented by Supervisor Matt Haney. Haney has backed the Embarcadero project and Navigation Centers in general. But he’s hinging his continued support of the seawall lot project on conditions that he’s discussed with Breed’s office, which he said was receptive to his proposals.

Specifically, Haney wants assurances that the center will focus on the waterfront’s homeless population and not bring in homeless people from other neighborhoods. He’s also asked for flexibility in the length of the center’s lease, round-the-clock security and indoor recreational facilities to minimize loitering during the day.

He’s also seeking a promise from the mayor to find at least one other neighborhood to build shelters in, apart from districts Six, Nine and 10, where homelessness services are clustered.

The three port commissioners present after public comment ended appeared to be in favor of the center. Commissioner Victor Makras said so explicitly, with the caveat that the city define how long the center would remain open.

“Everyone here in this city is responsible for this homelessness problem. We have to chip in and make some sacrifices,” said Willie Adams, the commission’s vice president. “You can be against anything. But you have to come up with some thoughtful, intelligent solutions.”

“Everybody needs to keep an open mind and give it a chance,” Port Commission President Kimberly Brandon said.

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