The Great Fluoride Debate of 2012 finally hit Portland on Thursday, with more than 275 people piling into City Hall to argue their case or simply listen to the occasional back-and-forth banter.

With approval already certain — a majority of the Portland City Council says it will vote next Wednesday in favor of fluoridating local drinking water — the 6-plus-hour marathon session served more as an opportunity to share convictions than to sway opinions.

But Thursday marked the first time city leaders heard public testimony, and listen they did.

A dozen health care professionals and community advocates dominated the stage, supplying city leaders at the outset with facts and figures supporting fluoride benefits and suggesting tooth decay should improve by at least 25 percent if approved locally.

Randy Leonard, the first city commissioner to support the fluoride effort, set the tone by saying 204 million Americans already drink fluoridated water and Portland needs to stop being an outlier.

“It’s time for Portland to join every other large city in the United States,” said Leonard, who worked with advocates behind the scenes before news of the effort became public four weeks ago. Leonard wants to fluoridate by March 1, 2014, two months before opponents hope to force a public vote through an initiative effort.

But opponents — who signed up to testify in a 3-to-2 ratio compared with supporters — continued their plea Thursday to maintain the fluoride-free status quo. They worried about impacts to people with thyroid problems and argued that medical treatment should be an individual choice. At the very least, they said, let the public vote, as Portlanders have four times since 1956. Residents voted in favor just once, in 1978, but overturned the decision in 1980.

“This is a blatant violation of due process,” said Kimberly Kaminski, who is leading the opposition group.

Portland’s water supply serves more than 900,000 people in Portland, Gresham, Tigard and Tualatin and other areas. The suburbs, which buy Portland water through wholesale contracts, have expressed displeasure with the city’s lack of public process.

The 2 p.m. hearing kicked off when a woman with a infant strapped to her chest interrupted Leonard nine minutes in, telling the City Council that fluoridation should be a parent’s choice. Mayor Sam Adams asked security to remove her in one of the hearing’s only public outbursts, although Adams later told onlookers to stop hissing.

Medical professionals asserted that concerns about lowered IQs, based on tests in China and Iran, had been “distorted and used as a fear tactic.” They said issues of fluorosis — white spotting of the teeth — were “very mild” at the optimal level being considered by Portland. They said it was “chaotic” for citizens to read medical reports and try to summarize the meaning.

“This is our defining moment to make a difference,” said Dr. Ken Wright, executive director of the dental program at Kaiser Permanente.

Their presentation stretched more than two hours, in part because technical problems prevented the hearing from being televised for a time, prompting Adams to delay testimony for about 30 minutes.

When Commissioner Nick Fish spoke up just after 4 p.m. to say he wanted to explain his support before leaving for another event, Commissioner Dan Saltzman interrupted to ask why hundreds of people had to keep waiting.

“I’m waiting for public testimony,” Saltzman said.

The hearing drew one of the biggest crowds to City Hall in years, with about 190 people in the City Council chamber, about 75 more in two overflow rooms at City Hall, and about a dozen at an auditorium in the neighboring Portland Building. That ranked it with the city’s Joint Terrorism Task Force decision, stadium renovations for the Portland Timbers and the ill-fated attempt to rename Interstate Avenue.

By 4:54 p.m., Fish had quietly left, and only 12 people from a list of 229 had testified.

In one of the city’s overflow rooms, where the hearing was being shown on television, Dr. Carlos Crespo waited patiently. As director of the School of Community Health at Portland State University, Crespo was recruited by proponents to speak. But after standing by for nearly 3 1/2 hours, keeping himself busy with work he brought along, Crespo worried about missing a dinner appointment.

“It’s nice to hear what people accept as truth,” Crespo said. “I’m enjoying it even if I don’t get a chance to talk. I’m enjoying the spectacle.”

In the other overflow area, Gresham resident Laureen Lucero said she doesn’t support fluoride and is disturbed that Portland officials didn’t communicate with wholesale customers.

She sat with her husband and granddaughter toward the back of the room, watching testimony unfold on television. They came just to listen.

“I was hoping to sit in the chambers and look them in the eye,” she said. “But we can’t do that. So here we are.”

By 7 p.m., the waiting list to testify still numbered 190-plus. Adams announced he was briefly leaving for an event at the Convention Center, where he was receiving an award.

At 8 p.m., with about two dozen people remaining to speak, Adams praised onlookers for their patience. One man responded, “Thank you for your endurance.”