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Boston’s 12 mayoral candidates, in their first televised debate, clashed repeatedly Monday over city policies and threw occasional jibes at each other’s records, but the all-inclusive format deprived any of them from a breakout moment.

In the Suffolk University debate, co-sponsored by New England Cable News and the Boston Herald, voters got their first televised look at all the candidates on stage at once. The format brought into sharp relief the challenge to voters trying to sift through a dozen candidates with diverse backgrounds toting differing messages from various corners of the city.

At times, perhaps inevitably with 12 candidates on the stage, the debate erupted in cacophony as candidates talked over each other.

But the hourlong televised format also allowed several candidates to be probed on components of their records that voters just tuning into the race, with a preliminary election on Sept. 24, would hear for the first time.

State representative Martin J. Walsh, who has strong support from unions, took fire over whether his history as a labor leader would render him ineffective as a chief executive when negotiating with public employee unions.

“I didn’t think I’d get a question on unions,” the Dorchester lawmaker cracked. “I haven’t heard it the whole race.” Saying he would “take an issue at a time,” Walsh pointed to union-bucking votes he had taken on charter schools, education reform, and pension crackdowns.

When Walsh’s Savin Hill neighbor, former health care and construction executive Bill Walczak, was asked whether he thought Walsh would be able to negotiate effectively with unions, his answer was crisp: “No, I don’t.”

Walsh was gentler toward Walczak when the latter came under moderator scrutiny for the severance package he received when leaving Carney Hospital, where he was president for 14 months.

“We were sad to see him go, because he was a great president of the hospital,” Walsh said.

More congenial neighborhood fraternity was evident between City Councilor John R. Connolly and Suffolk County district attorney Daniel F. Conley, a fellow West Roxbury resident, when Conley’s decision to send his children to private schools drew repeated questions from co-moderator Joe Battenfeld. Connolly, springing to Conley’s defense, called the questioning unfair.

But much of the debate was consumed by several hopefuls vying for attention.

At one point, Conley brandished a printed copy of his jobs plan, trying to draw moderators’ attention.

Later, an exchange over public safety, with much of the attention focused on Conley’s work in the prosecutor’s office, devolved into rhetorical chaos, as candidates repeatedly talked over one another, several gesticulating at once.

“Can I have the floor, Charles?” Conley asked radio station co-founder Charles Clemons, with whom he had been bickering. “You don’t talk over me in other forums, how about not tonight, OK?”

Conley fended off some of his rivals, who questioned his characterization of a rise in city shootings as moderate.

Former state representative Charlotte Golar Richie sounded her own note about urban violence, in an implicit swipe at both Conley and Mayor Thomas M. Menino, for whom Richie worked as housing chief.

“This is a tale of two cities … We need to shine a light on those neighborhoods, those pockets in our city, that are still wracked by violence,” she said.

Richie also denied involvement in a move by some of her supporters to shoulder other African American candidates out of the race, in an effort to consolidate support behind her candidacy. Richie said she could not be responsible for supporters going “rogue.”

“Anybody feels bullied, you should let me know about it,” she said.

City councilor Felix G. Arroyo, one of four candidates who opposes raising the cap on charter schools, was most impassioned when discussing education. The Jamaica Plain Democrat called the persistent chasm in achievement between students of different ethnic groups “the civil rights issue of our time.”

Asked whether Menino should throw the brakes on development as his fifth term nears its close, the candidates offered fractious replies. Conley and city councilor Rob Consalvo defended Menino’s right to press ahead, while others called for restraint. Connolly said “the next mayor has got to infuse some real transparency” at the Boston Redevelopment Authority.

Walczak went further, directing a plea at Menino: “Please stop.”

Non-profit executive John Barros, whose work along the Dudley Street corridor has drawn national plaudits, said the problem in city development lay in the amount of property being sold.

“It’s not about stopping the housing boom,” Barros said. Instead, he called for a separation within the development authority of planning and development decisions.

After the 60-minute TV session, the debate moved online for 30 minutes, where candidates fielded questions about whether they would keep on Boston Police Department Commissioner Ed Davis and how they viewed unpaid internships. Viewers who stayed with the televised broadcast were treated to a string of campaign ads, which, along with canvassing, mailers, and a seemingly endless spate of public forums, is how most of the candidates will reach voters in the final two weeks before the preliminary election narrows the field to two.