The Democratic primary campaign for Philadelphia mayor took a contentious turn Thursday with talk of a double-cross, and some heckling and cursing - in LOVE Park, of all places.

It unfolded like this:

Helen Gym, the education activist running for an at-large seat on City Council, appeared at a morning event at City Hall, where a collection of liberal groups criticized a trio of Main Line billionaires who are paying for television ads supporting State Sen. Anthony Hardy Williams' bid for mayor.

The trio, founders of the stock firm Susquehanna International Group, share Williams' enthusiasm for public school improvement, expansion of charter schools, and the use of tax dollars for private school tuition. They are spending money through an independent expenditure PAC called American Cities.

"These are three hedge-fund billionaires who are destroying education in a city they will never live in, for children they will never know," Gym declared. "And that's got to stop."

Williams, after a lunchtime election rally across the street at the park formally known as John F. Kennedy Plaza, asked School Reform Commission member Bill Green to speak to reporters. Green, a former councilman, said Gym met with Williams this week to discuss having her name on his sample ballot for the May 19 primary election, then attended the morning event.

"That is more than a little hypocritical, considering the fact that she founded a charter school, sent her children to charter schools, and her children are in special magnet schools," said Green, who has clashed with Gym over education.

Williams said he met with Gym and her campaign manager for about 90 minutes, discussing Susquehanna's founders and his record on education.

He said he later learned that Gym's campaign manager, Brendan McPhillips, is dating Jane Slusser, who is managing former City Councilman James F. Kenney's campaign for mayor. He said he now suspects a setup.

"I thought it was a productive, honest, candid conversation about how to bring people together," Williams said. "Apparently that was not the spirit in which she entered the conversation. She prepared and planned for this. And I'm disappointed and, frankly, disgusted by her duplicity."

McPhillips denied any duplicity, saying Williams' campaign asked for the meeting. He called Williams' attack on Gym "disappointing."

Gym's appearance at the event "wasn't personal at all," McPhillips said. "It was entirely substance-based, and she didn't mention [Williams] at all, or his campaign."

Then McPhillips called Green a "handpicked stooge" of former Gov. Tom Corbett to serve on the School Reform Commission.

Later in the day, Joel Greenberg, one of the Susquehanna founders, issued a statement saying the only motive for supporting Williams was to help children attending failing schools in Philadelphia.

"The most recent attacks against us are patently ridiculous," Greenberg said.

The Williams rally in LOVE Park drew a handful of liberal activists from the City Hall event, which had been hosted by the statewide group Keystone Progress.

Three of the activists held posters that read, "Stop billionaires from buying our next mayor."

The rally was attended by about 200 union members. Some exchanged curses and insults with the activists.

"Say hello to SIG for us," one of the activists heckled Williams.

"Say hello to Johnny Doc for me," responded Williams, referring to John Dougherty, leader of Local 98 of the International Brotherhood of Electrical Workers. Dougherty is supporting Kenney, and his union is helping finance Building a Better PA Fund, an independent expenditure PAC running pro-Kenney television commercials.

"If this is really about buying an election, she can't possibly be talking about outside interests and not about Johnny Doc, whom she clearly knows," Williams said of Gym.

215-854-5973 @byChrisBrennan