SOUTH PORTLAND, Maine — An ordinance that would prevent unrefined tar sands oil from being piped through the city passed the first of three hurdles in a special meeting Wednesday night.

The City Council voted 6-1 in a first reading of the “Clear Skies” ordinance.

More than 400 people gathered in the South Portland Community Center gymnasium for the council’s first vote on changes crafted by a special draft ordinance committee, which would prohibit “the bulk loading of crude oil onto marine vessels” in specific city zones and the construction of any buildings or equipment associated with that use.

“This is a historic night for us,” Rob Sellin of ordinance proponents Protect South Portland told councilors before the vote just after 11 p.m.

Wednesday’s crowd was color-coded: more than 100 opponents wore red T-shirts, decorated with American flags, that said “American Energy” on the front and “SoPo Jobs” on the back, while more than 300 others in support of the tar sands prohibition formed a wave of powder-blue in shirts proclaiming “Clear Skies.”

Many opponents were marine terminal employees, who fear the proposed ban on loading crude oil will jeopardize their jobs.

“This ordinance is poorly thought out and will work against our working waterfront and local economy,” said Tom Hardison, a Scarborough resident and longtime employee of Portland Pipe Line Corp.

Hardison added that the ordinance is “the first step to shutting down the oil industry in South Portland.”

But most councilors said they are unwilling to take a chance on tar sands, which is a common term for diluted bitumen, a thick form of crude oil mixed with clay and sand. Opponents of allowing tar sands into the city say transportation of the corrosive oil would increase the possibility of a leak in the aging Montreal-Portland pipeline, as well as intensify air and water pollution.

“If we don’t have our health or our safety, who cares if we have a job?” Councilor Patti Smith said.

Councilor Michael Pock, who cast the dissenting vote, previously voted against a temporary tar sands moratorium enacted last December.

“This whole DOC when it went through wasn’t done right; it was doomed to fail from the beginning,” Pock said.

City councilors formed the special committee in January to devise an approach to prohibit tar sands from the city, after a citizen-initiated ordinance was narrowly defeated in a referendum last November. It would have prevented Portland Pipe Line from reversing its pipeline to import Canadian tar sands and load the product on ocean-going tankers in South Portland.

Portland Pipe Line has historically exported crude oil to Canada. It had an air emissions license to reverse the pipeline flow to import into the city, but it relinquished the permit shortly before the November vote.

Company representatives have said there are no concrete plans to reverse the flow, but several oil industry representatives have said they oppose the recommended ordinance because it limits the potential expansion of their business. If the ordinance receives final passage, the city expects legal challenges.

“Sprague [Energy] is looking ahead, but we’re fully invested in ensuring the waterfront industrial businesses are allowed to compete and innovate,” Burt Russell, the company’s vice president of operations, said in an interview Tuesday.

More than 60 people spoke during Wednesday’s public comment period, with a 9-to-1 of speakers supporting the ordinance changes.

Several councilors said they voted against the tar sands ban last November because it was too broad, but the six who voted in favor Wednesday said the draft ordinance committee’s “surgical approach” — as committee member Russ Pierce described it — would adequately walk the line.

Mayor Jerry Jalbert also noted, when and if the ordinance passes, it can still be revised.

“If we find some parts are too restrictive, there are things we can change in the future,” Jalbert said.

The City Council also initially approved lifting the moratorium enacted in December “on development proposals involving the loading of oil sands/tar sands products onto marine tank vessels.”

The council was scheduled to have its first vote on the ordinance changes in their regular meeting at City Hall on July 7, but an unexpectedly large public turnout overwhelmed the 96-seat council chambers, forcing a postponement to Wednesday’s larger venue.

The proposed ordinance changes will go to the planning board on July 15 for a public hearing and recommendation. That meeting will be 7:30 p.m. at the South Portland High School Lecture Hall.

Final council action is scheduled for July 21.