CHICAGO (AP) — Chicago teachers went on strike Monday for the first time in 25 years after their union and district officials failed to reach a contract agreement despite intense weekend negotiations that the union said were productive but still failed to adequately address issues such as job security and teacher evaluations.

The two sides were not far apart on compensation, but were on other issues, including health benefits — teachers want to keep what they have now — and a new teacher evaluation system based partly on students' standardized test scores, Chicago Teachers Union President Karen Lewis said.

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

Mayor Rahm Emanuel condemned the union's decision, and said the negotiations could be resolved if the two sides kept talking, "given how close we are."

"This is not a strike I wanted," Emanuel said. "It was a strike of choice ... it's unnecessary, it's avoidable and it's wrong. "

More than 26,000 teachers and support staff were expected to hit the picket lines early Monday, while the school district and parents carried out plans for keeping nearly 400,000 students safe and occupied while classes remain empty in the coming days in the nation's third largest school district.

Both Emanuel and union officials have much at stake. The walkout comes at a time when unions and collective bargaining by public employees have 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.

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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.

School officials said they will open more than 140 schools between 8:30 a.m. and 12:30 p.m. so children can eat lunch and breakfast in a district where many students receive free meals. The 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. But it's not clear how many families will send their children to the added programs.

"They're going to lose learning time," said Beatriz Fierro, whose daughter is in the fifth grade on the city's Southwest Side. "And if the whole afternoon they're going to be free, it's bad. Of course you're worried."

Eric Ferrer, a cook, said his children can stay home Monday with his wife, who works in a store. But if the strike goes more than one day, they would have a problem — one that he sees no way to solve.

"My wife is off tomorrow, (so) we can keep them at home," said Ferrer, as he sat in a McDonald's restaurant on the city's Southwest Side with his wife and their 8-year-old son and 3-year-old daughter. "She works the next day (and) so do I."

School board President David Vitale first announced Sunday night that talks had broken off, despite the school board offering what he called a fair and responsible contract that would cover four years and meet most of the union's demands. He said the talks with the union had been "extraordinarily difficult."

Emanuel said the district had offered the teachers a 16 percent pay raise over four years, doubling an earlier offer.

Lewis said she would not prioritize the issues, saying that they all were important to teachers.

That included concern over a new evaluation that she said would be based too heavily on students' standardized test scores, which she said would be unfair to teachers because it could not adequately account for outside factors that affect student performance, including poverty, violence and homelessness.

She said the evaluations could result in 6,000 teachers losing their jobs within two years.

City officials said they did not believe that was true, but said the union would not tell them how they came to 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 mandated by state law but "was not developed to be a hammer," but to help teachers get better.

Emanuel said the union should have postponed the strike because an agreement is close. He also said his negotiating team would be available all night if the union was willing to talk, but Lewis said negotiations would resume Monday.

The strike is the latest flashpoint in a very public and often contentious battle between the mayor and the union.

When he took office last year, Emanuel inherited a school district facing a $700 million budget shortfall. Not long after, his administration rescinded 4 percent raises for teachers. He then asked the union to reopen its contract and accept 2 percent pay raises in exchange for lengthening the school day for students by 90 minutes. The union refused.

Emanuel, who promised a longer school day during his campaign, then attempted to go around the union by asking teachers at individual schools to waive the contract and add 90 minutes to the day. He halted the effort after being challenged by the union before the Illinois Educational Labor Relations Board.

The district and union agreed in July on how to implement the longer school day, striking a deal to hire back 477 teachers who had been laid off rather than pay regular teachers more to work longer hours. That raised hopes the contract dispute would be settled soon, but bargaining continued on the other issues.