Activist groups are demanding Mayor Martin J. Walsh bring more minorities to a fire department that is over 70 percent white — a figure mayoral challenger Tito Jackson quickly pounced on.

“I am very troubled and disappointed, but not surprised, by the findings of the Lawyer’s Committee that under Mayor Walsh diversity in the Boston Fire Department has actually become worse, not better, despite his promises,” Jackson said in a statement, adding he would create a chief diversity officer exclusively for the fire department.

A spokeswoman for Walsh said the mayor has “been working towards a BFD that better reflects the population of the city it protects and serves.”

The Boston Society of Vulcans, a black firefighter advocacy group, and the Lawyer’s Committee for Civil Rights and Economic Justice sent Walsh and fire Commissioner Joseph Finn a letter yesterday calling for a meeting with both men and for the department to improve diversity in its ranks. A spokesman for Finn said the commissioner was out of state at a conference and had not seen the letter.

The letter says departments that do not use available options to diversify their workforces could be open to lawsuits, and Vulcans Vice President Darrell Higginbottom would not rule out that possibility.

“A lawsuit is an option,” Higginbottom said. “The mayor often cites goals of diversifying city government; I hope we can work together before coming to a lawsuit.”

Walsh has cited increased diversity at City Hall during his time as mayor, but only nine black firefighters — 5 percent of total hires — have joined the fire department during his tenure, while 157 white firefighters account for 88 percent of new recruits.

The letter calls for Walsh and Finn to create a cadet program for the fire department, involve the Vulcans in the hiring process, recruit and promote employees that will encourage minority outreach and extend the residency preference requirement from one to three years.