CHICAGO – Thousands of teachers walked off the job Monday in Chicago's first schools strike in 25 years, after union leaders announced that months-long negotiations had failed to resolve a contract dispute with school district officials by a midnight deadline.

The walkout in the nation's third-largest school district posed a tricky challenge for the city and Mayor Rahm Emanuel, who said he would push to end the strike quickly as officials figure out how to keep nearly 400,000 children safe and occupied.

"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."