Citing disagreements over health benefits, evaluations and job security, Chicago's teachers' union went on strike as of Monday morning for the first time in 25 years.

"We know a strike is really going to be painful," union delegate Jay Rehak told The Chicago Tribune. "People will be hurt on both sides. But in the end, it's like saying, 'I'll be bloodied and you'll be bloodied, but at least you'll know not to bully me again.'"

Local school board president David Vitale said the city had offered teachers a 16 percent pay raise over four years, along with other benefits packages, as part of a proposal that changed 20 times over the course of negotiations.

"This is about as much as we can do," Vitale said. "There is only so much money in the system."

In a statement late Sunday night, union president Karen Lewis acknowledged that the two sides had made progress in salary negotiations, but said disagreements over health benefit costs, job security and the way teachers were evaluated remained.

WLS-TV reported that teachers and supporters were gathering at Chicago Public Schools headquarters as early as 5 a.m. Monday to start picketing, with more than 26,000 faculty and support staff expected to participate.

As the strike began, the city's school district opened 144 schools on a "last resort" half-day basis for the 357,000 students affected, with other community organizations and 78 of the city's libraries also availing themselves for use.

"The choice by the leadership of the Chicago Teachers Union to strike is an incredible burden on our families and children," CPS CEO Jean-Claude Brizard said in a statement, according to WMAQ-TV. "I want to thank Chicago's community and faith leaders for coming together to support our students in what will be a difficult time."

Mayor Rahm Emanuel told WBBM-TV he considered the strike unnecessary, citing progress in negotiations regarding teachers' compensation. He also downplayed criticism at not being personally involved in the process; the Tribune reported that according to several sources, Emanuel was discouraged from taking part because of "too much bad blood" between himself and Lewis.

"It's not about my presence, it's about reaching agreement," he said. "As I believe, this is an honorable deal. It's an honest compromise between both parties, without anybody having to compromise principle."

WLS-TV's report on the strike, aired Monday morning, can be seen below.

Update: Headline has been corrected

Update, 12 p.m. EST: Republican presidential nominee Mitt Romney released a statement criticizing the strike.

"Teachers unions have too often made plain that their interests conflict with those of our children, and today we are seeing one of the clearest examples yet," Romney said. "President Obama has chosen his side in this fight, sending his Vice President last year to assure the nation’s largest teachers union that 'you should have no doubt about my affection for you and the President's commitment to you.' I choose to side with the parents and students depending on public schools to give them the skills to succeed, and my plan for education reform will do exactly that."