"At this meeting, an individual in attendance made an anti-Semitic comment," Johnson said in a statement. "This comment was totally inappropriate, and will not be tolerated."
Johnson did not attend the meeting, but a staffer from his office was present.
Ori Feibush, a Point Breeze developer who unsuccessfully challenged Johnson in last year's Democratic primary, said a member of Concerned Citizens was leading the opposition to builders who were making presentations at South Philadelphia HOMES, the community organization that hosted the meeting.
"This was not one person with a screw loose," Feibush said. "This was a mob mentality."
Feibush, who is Jewish, did not present any projects at the meeting. He was attending as a neighbor.
One comment he said he heard was, "Why don't you go back to Israel?"
"It was hostile," said Kathy Knese, a South Philadelphia resident who heard some of the remarks.
One attendee, who asked not to be identified, said he heard someone shout, "Go back to Lakewood," a town in New Jersey with a large concentration of Orthodox Jews.
Some of the presenters were Jewish. At least one was wearing a yarmulke, a brimless cap worn by some Jews.
"I've experienced plenty of anti-Semitism in that neighborhood, but last night was by far the most vitriolic I've ever seen," Feibush said. "It wasn't one person. It was not even remotely veiled. It was completely and utterly focused on those that were presenting. It was abhorrent."
The meeting was brought to an end after the head of South Philadelphia HOMES was notified that a person had made a comment that was widely perceived to be anti-Semitic, according to an attendee. Outside, a shouting match ensued, some captured on video by Christopher Sawyer, who runs the Philadelinquency blog and ran for sheriff last year.
Concerned Citizens, whose leaders have clashed with Feibush and others, has sought to block some developments, saying they will attract "yuppies" and displace lower-income minority residents by driving up property values.
In addition to the Human Relations Commission, Johnson said, he had notified the Planning Commission, which recognizes Concerned Citizens as a registered community organization. Developers seeking zoning variances must present their plans to such organizations.
Claudia Sherrod, head of South Philadelphia HOMES, also sent a letter to the Planning Commission on Tuesday, asking it to take action against the "out of control" Concerned Citizens.
Concerned Citizens did not respond to a request for comment. But on Tuesday, the organization sent an email to Feibush - much of it in capital letters - saying that he was lying about the community meeting.
"Don't try to twist what was said at the zoning meeting last night or play victim," the email stated (in lowercase for readability). "You have been plotting and scheming about us on Facebook since Sunday." Feibush's Facebook page showed no posts before the meeting since Feb. 11.