Despite blunt warnings from Mayor Carlos Gimenez that their decision would force layoffs of hundreds of police and corrections officers, Miami-Dade commissioners Thursday narrowly refused to force two employee unions to contribute an additional 5 percent of their pay toward health insurance.

Facing a chamber packed with police officers, the commissioners voted 7-6 against settling a labor impasse between the county and the Dade Police Benevolent Association by forcing the additional concession on 5,400 police and corrections officers.

The panel also refused to impose the controversial 5-percent giveback on its professionals and supervisors, who are represented by the Government Supervisors Association of Florida OPEIU Local 100. That move spells an almost-certain wave of layoffs for them as well.

“It’s an absolute lack of political courage by a board that wants to have its cake and eat it too,’’ the mayor said at the conclusion of a nine-hour meeting. “I’m disgusted at what happened today.’’

The mayor estimated that at least 154 police officers and 145 corrections officers would have to be laid off as the county scrambles to close a budget gap it had expected to fill through additional employee concessions. The county will also likely have to close a corrections facility.

The mayor said he and his staff would meet Thursday night to begin hashing out how to fill the $35-million budget gap that he had envisioned eliminating with the additional health-care contribution. He said other options have already been exhausted.

The mayor said commissioners knew that painful employee concessions were needed when they voted in September to approve his budget plan, which reduced the property-tax rate. “When it came down to making the tough decision, they buckled,’’ he said.

Commissioners offered various explanations for their reluctance to defy the unions. Commissioner Jose “Pepe’’ Diaz said he couldn’t force the concession on police after attending funerals of slain officers and knowing how tough their jobs are.

At one point, Diaz asked the officers in the audience if they would rather swallow the concession or reject it and see fellow officers laid off. “Do it!’’ several officers shouted, with others echoing support of the layoffs.

The tortured vote by commissioners, who second-guessed their decision before sticking by it, came after nearly seven hours of pleas by the labor unions that are traditionally among the politicians’ most reliable fundraisers and voting blocks.

Half of the commissioners — those in odd-numbered districts — face re-election contests in August.

The seven commissioners who voted against the increased healthcare contribution were Bruno Barreiro, Diaz, Barbara Jordan, Jean Monestime, Javier Souto, Xavier Suarez and Chairman Joe Martinez. Voting for it were Lynda Bell, Esteban Bovo, Sally Heyman, Dennis Moss, Rebeca Sosa and Audrey Edmonson.

Edmonson said she supported the measure because she did not want to see police officers laid off. “I cannot vote for adding to the unemployment’’ rate, she said.

The PBA, which bargains for police and corrections officers, and the GSAF, representing nearly 4,600 professional employees and supervisors, recently approved new labor contracts that include significant cost savings to the county, but the parties hit an impasse on doubling the health insurance contribution to 10 percent of base pay.

John Rivera, president of the PBA, made an impassioned argument against the extra 5 percent healthcare contribution, asserting the county’s plan is on shaky legal ground and an unfair burden on police.

Relatives of slain officers made emotional presentations urging support of the union’s stance. Among those attending was Robert Haworth, the father of deceased Officer Amanda Haworth, and Debbie Castillo, widow of Officer Roger Castillo, who were killed in the line of duty last year.

Commissioners and union leaders criticized the mayor for focusing too heavily on employee concessions instead of finding savings elsewhere in the $4.4 billion operating budget.

In addition, Commissioner Jordan took aim at the mayor for moving slowly in reaping savings from his much-ballyhooed reorganization, which reduced the number of county departments to 25 from 42.

Mayor Gimenez reiterated that his reorganization plan is ongoing and will take time. “I’m not going to rush into something and do it the wrong way,’’ he told the restive commission.

Three other unions — representing solid waste employees, water and sewer workers, and general county employees, have reached tentative contracts but also are at impasse on the healthcare contribution. If employees ratify those agreements in coming days the commission will again have to decide whether to impose the extra 5 percent on those groups.

Solid waste employees — represented by the American Federation of State County and Municipal Employees Local 3292 — are set to vote Friday on a tentative agreement. Water and sewer workers, who belong to AFSCME Local 121, are set to vote Monday and general county employees, represented by AFSCME Local 199, will weigh in on their tentative pact on Jan. 20.

The county’s labor strife is far from over. On Wednesday, members of the Transport Workers Union Local 291 rejected a tentative agreement by a vote of 820 to 660. That means further delays as the parties return to the table to haggle over a new agreement.

The county’s budget plan had envisioned obtaining $239 million in various labor concessions from its 10 unions, but the mayor said last month that about a quarter of that won’t be achieved because of delays in obtaining the cost-saving labor contracts.