A group that represents 18,000 employees at Los Angeles City Hall warned Tuesday it will sue if the City Council enacts a plan to roll back  pension benefits and boost the retirement age to 65.

With the pension plan up for a vote Tuesday, lawyers for the Coalition  of L.A. City Unions sent a letter to Mayor Antonio Villaraigosa threatening a lawsuit and warning the reductions in  retirement benefits will cause "irreparable harm."

Hundreds of city  employees have scheduled a morning rally at City Hall, where they plan to argue that Villaraigosa  and the City Council have launched an attack on unions similar to those  waged in Wisconsin, where Republican Gov. Scott Walker has battled unions  for much of his first term.

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