SAN JOSE — Police and city leaders Thursday announced mandatory overtime and the possible cancellation of an academy for new officers in a department where they say recruitment and retention is still reeling from city efforts to reduce costly pensions.

The announcement came a day after a taxpayer group launched a legal effort to maintain Measure B pension reforms voters approved in 2012 that union leaders blame for shrinking the department to the breaking point.

“If Measure B is not invalidated and this pattern continues, we’ll have no choice than to use overtime further to ensure that when someone calls 911 their call is answered,” said police Chief Eddie Garcia during a news conference.

Garcia and police union officials were joined by Mayor Sam Liccardo, City Manager Norberto Dueñas and other city leaders in a rare showing of unity to fend off the legal challenge by former Councilman Pete Constant, billionaire local Republican leader Charles Munger Jr. and the Silicon Valley Taxpayers Association to a proposal to replace Measure B with a more generous pension settlement negotiated with city unions.

The group filed legal papers Wednesday to block the city’s push to nullify Measure B. The city is asking a judge to overturn the measure based on a “procedural error” — city leaders didn’t fully bargain with unions before putting the measure on the ballot. This move allows city leaders to replace Measure B with the negotiated settlement reached with unions last year.

Constant says his intervention isn’t to challenge the merits of the settlement, which offers a more generous pension to new hires but locks in some savings from Measure B such as eliminating retiree bonus checks. He said he’s disputing the city’s process: Since voters overwhelmingly approved Measure B in 2012, Constant says, any changes must be approved by them.

“This is about respecting the law and respecting the will of the voters,” Constant said.

But the long and painful fight over pensions in San Jose took its toll on the Police Department, Garcia argues, and now all officers will be forced to work overtime as the agency continues a struggle with meeting minimum patrol levels.

After parsing out trainees, injuries and other staff on leave, the Police Department is fielding 839 street-ready officers and is stuck filling 252 10-hour shifts each week with overtime, Garcia said Thursday.

Until Thursday, overtime had become compulsory in spurts, with some investigative units and other divisions being partly spared. That apparently ends in the coming weeks.

Additionally, the department announced that it is considering whether to hold another police academy after the current crop yielded just seven recruits, a far cry from the usual 60 per class.

Police leaders thought the dark days were behind them when they struck a settlement on Measure B last summer with City Hall. Now, they say the legal challenge threatens to derail that progress.

“We do not need outside influences impacting this process and, by extension, the safety of our police officers,” Garcia said.

Constant, who retired from the San Jose Police Department in 2000 but now lives in Roseville, took offense to being called an “outsider.”

“I find it preposterous that the mayor and police chief referred to me as an outsider when I spent 29 years serving the residents of San Jose directly,” Constant said after the news conference. “If I’m an outsider, who’s an insider? For God’s sake, I helped author this ballot measure.”

And while city and union officials are drafting a November ballot measure to require voter approval for future pension increases and other safeguards, voters won’t be asked to approve the settlement framework. Constant says that entire accord — not just bits and pieces — must legally go back to voters.

Although Liccardo said during his recent State of the City speech that voters will be asked “to approve a ballot measure to secure” pension savings, Constant says that’s inaccurate. Voters will only be asked to approve prohibiting retroactive pension increases, requiring voter approval for benefit increases and requiring actuarial soundness in the pension plan.

Liccardo, who expressed disappointment in Constant’s move, said his job is to ensure the Police Department is rebuilt quickly.

“People elected me to fix problems, people elected me to confront crises,” Liccardo said during the news conference. “And we have a crisis today with our police staff.”

City Attorney Rick Doyle said the group’s push to be an opposing party in the city’s case to invalidate Measure B is “out of the ordinary” but merely a delay and not a serious legal challenge.

“I see it as a nuisance, and we should be pretty successful in getting it thrown out,” Doyle said. “This is trying to create five more years of litigation and uncertainty. Let’s come to an agreement, get the courts to sign off on it, be done with the lawyering and get on with the business of running the city.”