CHICAGO —  For the first time in a quarter century, Chicago teachers walked out of the classroom Monday, taking a bitter contract dispute over evaluations and job security to the streets of the nation’s third-largest city — and to a national audience — less than a week after most schools opened for fall.

The walkout forced hundreds of thousands of parents to scramble for a place to send idle children and created an unwelcome political distraction for Mayor Rahm Emanuel. In a year when labor unions have been losing ground nationwide, the implications were sure to extend far beyond Chicago, particularly for districts engaged in similar debates.

“This is a long-term battle that everyone’s going to watch,” said Eric Hanuskek, a senior fellow in education at the Hoover Institution of Stanford University. “Other teachers unions in the United States are wondering if they should follow suit.”

The union had vowed to strike Monday if there was no agreement on a new contract, even though the district had offered a 16 percent raise over four years and the two sides had essentially agreed on a longer school day. With an average annual salary of $76,000, Chicago teachers are among the highest-paid in the nation, according to the National Council on Teacher Quality.

But negotiators were still divided on job security measures and a system for evaluating teachers that hinged in part on students’ standardized test scores.

The strike in a district where the vast majority of students are poor and minority put Chicago at the epicenter of a struggle between big cities and teachers unions for control of schools.

Emanuel, who has sought major reforms while also confronting the district’s $700 million budget shortfall, acknowledged his own fight with the union, even as he urged a quick resolution.

“Don’t take it out on the kids of Chicago if you have a problem with me,” he told reporters Monday.

As negotiators resumed talks, thousands of teachers and their supporters took over several downtown streets during the Monday evening rush. Police secured several blocks around district headquarters as the crowds marched and chanted.

The protesters planned to rally through the evening at an event that resembled a family street fair. Balloons, American flags and homemade signs hung above the crowd.

Teacher Kimberly Crawford said she was most concerned about issues such as class size and the lack of air conditioning.

“It’s not just about the raise,” she said. “I’ve worked without a raise for two years.”

The strike quickly became part of the presidential campaign. Republican candidate Mitt Romney said teachers were turning their backs on students and Obama was siding with the striking teachers in his hometown.

Obama’s top spokesman said the president has not taken sides but is urging both the sides to settle quickly.

Emanuel, who just agreed to take a larger role in fundraising for Obama’s re-election, dismissed Romney’s comments as “lip service.”

But one labor expert said that a major strike unfolding in the shadow of the November election could only hurt a president who desperately needs the votes of workers, including teachers, in battleground states.

“I can’t imagine this is good for the president and something he can afford to have go on for more than a week,” said Robert Bruno, a professor of labor and employment relations at the University of Illinois at Chicago.

For two decades, contract agreements have slowly eroded teachers’ voices, Bruno said.

“But this signals to other collective bargaining units that the erosion of teachers’ rights isn’t inevitable. They (the union members) are telling them, ‘You don’t have to roll over.”

The union has done so in large part by making the most of one of the biggest sources of friction: teacher evaluations.

In Chicago, union President Karen Lewis suggested the city’s proposal could put thousands of teachers’ careers at risk because the evaluation system relies too heavily on standardized test scores and does not take into account such factors as poverty, violence and homelessness.

Teachers “have no control over those scores,” said union coordinator John Kugler.

The union feared 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.

The strike involving more than 25,000 teachers meant no school for 350,000 students and raised the worries of parents who were concerned not just about their kids’ education but their safety. Gang violence in some parts of the city has spiked in recent months.

“They’re going to lose learning time,” said Beatriz Fierro, whose daughter is in the fifth grade. “And if the whole afternoon they’re going to be free, it’s bad. Of course you’re worried.”

In response, Police Superintendent Garry McCarthy said he would take officers off desk duty and deploy them to deal with any protests as well as the scores of students who might be roaming the streets.

The district staffed 144 schools with non-union workers and central office employees for half the day so students who are dependent on school-provided free meals would have a place to eat breakfast and lunch.

One after another, parents refused to leave their children at unfamiliar schools where they would be thrown together with kids and supervising adults they may never have met.

April Logan arrived at the Benjamin Mays Academy on the city’s South Side with the intention on dropping her 5-year-old daughter off but then thought better of it.

“I don’t understand this, my baby just got into school,” she said, just before turning around and walking home with the child.

Some students expressed anger, blaming the district for interrupting their education.

“They’re not hurting the teachers. They’re hurting us,” said Ta’Shara Edwards, a student at Robeson High School on the city’s South Side. She said her mother made her come to class to do homework so she “wouldn’t suck up her light bill.”

However, many parents appeared sympathetic.

Sarah Allen, whose daughter is a seventh-grader, said she saw Emanuel at the Democratic National Convention “listening to Bill Clinton talk about compromise and cooperation.”

But Emanuel seems to have “built a lifestyle around being a bully,” she said. “And it’s one thing to be a chief of staff and another to be a leader.”

Emanuel, who has engaged in a public and often contentious battle with the union, is not personally negotiating, but he’s monitoring the talks through aides.

Not long after his election, the mayor’s office rescinded 4 percent raises for teachers. Then he 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, a request the union turned down.

Emanuel, who promised a longer school day during his campaign, 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 a deal to implement the longer school day, crafting a plan 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 stalled on the other issues.