City councilmembers scrambled Thursday to mobilize opposition against plans for the country’s first supervised injection site, causing a political dilemma for Mayor Jim Kenney, one of the initiative’s most prominent supporters, and helping to unravel the scheduled opening of a facility in South Philadelphia next week.

Councilmember David Oh filed proposed legislation that would make it all but impossible to open additional injection sites in the city.

Council President Darrell L. Clarke said he was involved in discussions about litigation to stop the nonprofit operator, Safehouse, from opening its facility in South Philadelphia next week, a step that became unnecessary Thursday evening when the owner of the building where the site was to open pulled out of the plan. And several lawmakers gave impassioned speeches opposing the plan, as well as the administration’s handling of it.

“There’s likely a majority of councilmembers who either are not happy with the process or don’t necessarily support the whole premise of safe injection sites,” Clarke said after Council’s first meeting since the plan was announced. “I don’t fundamentally understand how you get to a point where you have a person stop using drugs by enabling them to use drugs in a quote-unquote safe manner.”

There is likely more than a simple majority of Council’s 17 members who fit that description. No members came forward to defend the plan or the mayor on Thursday.

Some councilmembers and other Philadelphia politicians have said they are open to using supervised injection sites as a strategy in the fight against the city’s opioid crisis. But anger with Safehouse and the administration’s handling of the rollout of the first site has unified an often-divided Council in opposition to the mayor.

The proposed location, which is near two preschools, leaked hours after a federal judge greenlighted the use of injection sites in Philadelphia, without warning to councilmembers or input from community groups.

“Whether you support these supervised injection sites or you don’t, you have to be upset at the process,” Councilmember Mark Squilla said. “What happened this week was just outright wrong.”

Kenney spokesperson Mike Dunn said misinformation drove opposition to the plan to open the site in the Constitution Health Plaza on Broad Street near Passyunk Avenue.

“The mayor and staff certainly have heard the concerns voiced today by Council members, and we’ve encouraged Safehouse to engage in additional community outreach,” Dunn said in a statement. “That said, it is important that the concerns of constituents not be exacerbated by false information.”

Overdose prevention sites, as the administration refers to them, have been successful in other countries, Dunn said. In Philadelphia, a site could save from 24 to 76 lives per year, he said, while providing “a venue so that public health professionals can connect individuals to drug treatment and other vital support services.”

Dunn also pushed back on a rumor, repeated by councilmembers Thursday, that plans for 20 supervised injection sites across the city were in the works.

“That is categorically false,” he said. “The number of sites needed would depend on the capacity of each site, and that has not been determined. What we do know is that this is very much a citywide problem, and the need to provide support services to those struggling with addiction is great. In short, the mayor would support as many as it takes to keep people from dying.”

Kenney on Wednesday said the only additional planned injection site that he was aware of is for a facility in Kensington, which is at the heart of Philadelphia’s opioid crisis.

Councilmembers weren’t buying the mayor’s message on Thursday.

“When you are a homeowner, and what is going in there will have an impact not just on your quality of life but on the value of what could be your only asset, you have a right to speak, and you have a right to have a role,” said Majority Leader Cherelle L. Parker, who for the first time Thursday said that she opposes any injection sites in the city.

Oh’s bill, which was written hastily and will likely be substantially amended if it advances, would require approval from Council and 90% of residents, businesses, and institutions within a one-mile radius of a planned injection site before a facility could be opened — effectively banning them.

Without offering details, Clarke said he has been involved in talks about suing to prevent the Safehouse facility from opening and to find a way to prevent future sites.

“There is a lot of movement and discussion around any potential legal action that could be taken to delay the opening of the facility,” Clarke said. “We will see over the next couple of days. Monday’s coming. Clock is ticking. And even if it does open, there will probably continue to be action taken to eliminate this site, and actually eliminate the premise of having safe injection sites in the city of Philadelphia.”