The union representing 4,100 janitors in buildings on L.A.’s Westside has reached a tentative contract agreement with building owners and janitorial service contractors, apparently averting a possible strike, union officials and a spokesman for L.A. Mayor Antonio Villaraigosa announced Monday afternoon.

Details of the contract settlement were set to be unveiled at a news conference late Monday afternoon in the Mayor’s press room at L.A. City Hall.

Service Employees International Union Local 1877, representatives of the Building Owners and Managers Association and several local janitorial service contractors had been in talks since the current janitors contract expired at the end of April. At issue besides demands for higher wages and benefits was what the union termed a two-tier pay system in which janitors in some buildings were paid more than others.

Last week, SEIU mounted several local demonstrations as contract talks hit a temporary impasse. This first round of labor action targeted 20 Westside buildings.

But last Thursday, Mayor Villaraigosa intervened and arranged a cooling off period. According to an advisory from Villaraigosa’s office, the two sides held talks throughout the weekend before reaching a tentative agreement early Monday afternoon.