Nearly 850 Chicago Public Schools employees received layoff notices Friday, hours after officials said they had identified $52 million in administrative and operational cuts to help close an estimated $1 billion deficit in the upcoming fiscal year.

Eliminating almost 100 central office positions and adopting more efficient building maintenance will help save $20.7 million, on top of $31.6 million in cuts announced earlier this year, officials said. They did not estimate how much the district might save through layoffs — most resulting from a Board of Education decision last month to close 50 schools and programs — because some workers will be eligible to reapply for other jobs.

CPS CEO Barbara Byrd-Bennett called the deficit "historic." She said the district will spend the next several weeks looking for more ways to trim expenses but will try to avoid cuts that would affect students.

"Next year's budget will not come without painful decisions, which is why we are making tough choices at central office in order to minimize impacts to our classrooms," Byrd-Bennett said in a written statement.

Officials say that since 2011, CPS has cut almost $600 million from the central office and programs that it funds.

The nation's third-largest school district has been struggling to trim its deficit, which it said was inflated by a $400 million increase in its annual pension payments, salaries and flat or declining revenue.

The layoffs included about 550 teachers — both from schools that are closing and struggling academically — along with teacher assistants, bus aides, custodians and others.

Mayor Rahm Emanuel promoted the school-closure plan, saying the buildings were underused and that closing them it would save the district hundreds of millions over 10 years and improve education. Chicago Teachers Union Staff Coordinator Jackson Potter called the layoffs and closures "part and parcel of a very intense series of attacks on our schools that will undermine the academic growth and development and the safety of our most vulnerable students."

He contends that Emanuel has refused to explore other options, including by demanding a more equitable state tax system and taking back tax revenue lost to tax-increment finance districts meant to promote development. Potter also said the union believes the layoffs "could be the tip of the iceberg" as individual schools assess their budgets.

Emanuel and Byrd-Bennett said CPS has 403,000 students in a system that has seats for more than 500,000. The school closures have sparked protests and lawsuits from opponents who say they disproportionately affect minority neighborhoods and will endanger children who may have to cross gang boundaries to get to a new school.

Officials have said students will be moved to schools that are performing better academically and that CPS will work with Chicago police and community groups to ensure students can get to and from their new schools safely.