Thousands of teachers walked off the job Monday in Chicago, the third-largest U.S. school district, as city officials prepared to look after thousands of students who could end up wandering unsafe streets.

Some 26,000 teachers and support staff were expected to join the picket after union leaders announced they were far from resolving a contract dispute with school district officials. City officials acknowledged that children left unsupervised -- especially in neighborhoods with a history of gang violence -- might be at risk, but vowed to protect the nearly 400,000 students' safety.

The walkout posed a tricky test for Mayor Rahm Emanuel, who said he would work to end the strike quickly.

"This is not a strike I wanted," Emanuel said Sunday night, not long after the union announced the action. "It was a strike of choice ... it's unnecessary, it's avoidable and it's wrong."

Contract negotiations between Chicago Public School officials and union leaders that stretched through the weekend were resuming Monday.

Among teachers protesting Monday morning outside Benjamin Banneker Elementary School on Chicago's South Side, eighth-grade teacher Michael Williams said he wanted a quick contract resolution.

"We hoped that it wouldn't happen. We all want to get back to teaching," Williams said, adding that wages and classroom conditions need to be improved.

Officials said some 140 schools would be open between 8:30 a.m. and 12:30 p.m. so the children who rely on free meals provided by the school district can eat breakfast and lunch, school district officials said.

"We will make sure our kids are safe, we will see our way through these issues and our kids will be back in the classroom where they belong," Emanuel said.

The school district asked community organizations to provide additional programs for students, and a number of churches, libraries and other groups plan to offer day camps and other activities.

Police Chief Garry McCarthy said he would take officers off desk duty and deploy them to deal with any teachers' protests as well as the thousands of students who could be roaming the streets.

Union leaders and district officials were not far apart in their negotiations on compensation, Chicago Teachers Union President Karen Lewis said. But other issues -- including potential changes to health benefits and a new teacher evaluation system based partly on students' standardized test scores -- remained unresolved, she said.

"This is a difficult decision and one we hoped we could have avoided," Lewis said. "We must do things differently in this city if we are to provide our students with the education they so rightfully deserve."

Emanuel and the union officials have much at stake. Unions and collective bargaining by public employees have recently come under criticism in many parts of the country, and all sides are closely monitoring who might emerge with the upper hand in the Chicago dispute.

The timing also may be inopportune for Emanuel, a former White House chief of staff whose city administration is wrestling with a spike in murders and shootings in some city neighborhoods and who just agreed to take a larger role in fundraising for President Barack Obama's re-election campaign.

Republican presidential nominee Mitt Romney said Chicago teachers are turning their backs on thousands of students and Obama is siding with the striking teachers.

Romney, in a statement released Monday hours before he was set to land in Chicago for fundraisers, said he is disappointed by the Chicago teachers' decision to walk out of negotiations. Romney, who has been critical of public employee unions, said he sides with parents and students over unionized teachers.

As the strike deadline approached, parents spent Sunday worrying about how much their children's education might suffer and where their kids will go while they're at work.

The school board was offering a fair and responsible contract that would most of the union's demands after "extraordinarily difficult" talks, board president David Vitale said. Emanuel said the district offered the teachers a 16 percent pay raise over four years, doubling an earlier offer.

Lewis said among the issues of concern was a new evaluation that she said would be unfair to teachers because it relied too heavily on students' standardized test scores and does not take into account external factors that affect performance, including poverty, violence and homelessness.

She said the evaluations could result in 6,000 teachers losing their jobs within two years. City officials disagreed and said the union has not explained how it reached that conclusion.

Emanuel said the evaluation would not count in the first year, as teachers and administrators worked out any kinks. Schools CEO Jean-Claude Brizard said the evaluation "was not developed to be a hammer," but to help teachers improve.