The Boston police patrolman’s union is in a tense standoff with Mayor Martin J. Walsh and police Commissioner William B. Evans over the city’s stalled body camera pilot program, after the mayor said he would require cops to wear the cameras if no one volunteers.

An agreement between the city and the union hashed out earlier this summer called for 100 volunteer officers to wear the cameras in a six-month pilot program — in exchange for $500 bonuses — but so far no cops have signed on as the program approaches its target start date of Sept. 9.

“It is a voluntary program, however, if officers don’t step up to do it or if we don’t get a sufficient amount of officers to do it, we’re going to be putting them out there on officers,” Walsh said yesterday, adding he would not up the bonus to encourage applicants.

“We’re making this a directive,” he said. “They have to wear them.”

The officers would be chosen at Evans’ discretion, according to BPD spokesman Stephen McNulty.

But Boston Police Patrolman’s Association President Patrick M. Rose told the Herald that goes against the deal the union reached with the department, which he says specifically states participants must be volunteers.

“The selection process must be from volunteers,” Rose wrote in an email to the Herald, adding that the union still supports that agreement.

“To require non-volunteers to participate in the program would clearly violate the agreement,” he said. “The BPPA would hope that the City and the Department would honor its written agreement with the BPPA concerning (body cameras).”

However, McNulty said officers’ refusal to take part was a deal-breaker itself.

“Not getting volunteers also violates the agreement,” McNulty wrote to the Herald.

Black leaders yesterday decried the lack of cops willing to wear body cameras — a key issue in minority communities in the wake of high-profile shootings by police. And they accused the union of using the issue as a gambit in contract negotiations. The union’s contract expired at the end of June.

“We are deeply disappointed by the union’s unwillingness to respond to what residents are calling for, with the adoption of this technology,” said Michael Curry, president of the NAACP in Boston.

“It has been clear that they are unwilling to evolve and thereby avoid what we are seeing across the country,” Curry said. “This issue of body cameras should not be used as a political carrot in their efforts to renegotiate their contract.”

Curry is calling on black and Latino officers to be the first to volunteer. But the head of the state’s minority officers association told the Herald they are concerned about whether police brass would back them up if problems arise.

Curry said he understands the fear of being the officer that no one wants to stand next to for fear of being recorded.

“But as we are fond of saying, courage will not skip this generation,” Curry said. “In the police department we need some courageous officers who are not afraid of more transparency and accountability. We believe that history will receive them kindly.”

Relations between police unions and Walsh and Evans have hit the boiling point in recent weeks. The BPPA and two other unions blasted the mayor and commissioner for not taking safety concerns seriously in a letter sent earlier this month, demanding heavy weaponry and other changes in the wake of the Dallas and Baton Rouge, La., shootings of police officers.

Walsh said he did not think the lack of volunteers was tied to those concerns, but to larger worries about change.

“I think the reluctance is the uncertainty of change,” the mayor said, “change across the board.”