Members of the unions that represent Atlanta firefighters, police officers and other city employees have filed a lawsuit to undo a significant piece of an overhaul to the city’s pensions that largely defined Mayor Kasim Reed’s first term.

Stephen Borders, president of the Atlanta Professional Firefighters union, said the lawsuit’s aim of rolling back the increased employee contribution to the pensions required by the 2011 overhaul — currently a 5 percent increase and possibly another 5 percent later for workers who were employed by the city then — is a matter of fairness and law.

The lawsuit filed Thursday in Fulton County Superior Court says the increase in the contribution is an unconstitutional breach in the city’s contractual obligations to employees and takes money out of employees’ pockets.

“It is something we felt was wrong from the moment we heard about it, but we haven’t had an avenue to attack it,” Borders said. “Our situation has been made worse because they have completely thrown out our pay structure and raises. Now any raise in your career is at the will of the City Council.”

Plaintiffs are hoping to gain class-action status for the suit, which says that through the increase in the contribution, employees will ultimately pay “over 50 percent more to purchase the same amount of retirement benefits to which they already were entitled.”

When city officials made the changes in the pension law, they said they were facing a $1.5 billion pension liability. Actuaries said that all of the components of the overhaul — including more reduced benefits for future employees — would save the government more than $270 million over 10 years.

“Now I’m breathing easier,” Reed said at the time of the overhaul, possibly his major achievement as mayor. “This is a good day because we have stopped the bleeding in Atlanta.”

John Bell, an attorney for union members in the suit, said Georgia courts have generally sided with employees when government officials have undercut their pension plans since a case involving a firefighters pension during the Great Depression.

The issue in this case will be whether the city was allowed to force existing employees to contribute more to their pensions, which undercuts the value of their compensation, Bell said. Lawyers for the city said the city could change the plan, Bell said, citing a notice he said was mailed to employees that the city was considering such a move.

“We don’t think sending somebody an after-the-fact notice that ‘We might break our promise’ lets you break your promise,” he said. “It is our position that a change in the cost of the pension is the same as a change in the amount of the pension.”

City officials do not comment on pending legislation.

Reports from the time said that employees initially agreed to the changes, which reduced the number of city layoffs.