Media playback is unsupported on your device Media caption Chicago teachers rally as strike begins

Teachers in Chicago have gone on strike for the first time in 25 years, prompting a showdown with Mayor Rahm Emanuel over a contract dispute.

As many as 26,000 teachers were expected to stay away, with picket lines forming around the city.

About 350,000 students were affected by the strike in the nation's third-largest district.

The walkout was called after all-day talks broke down on Sunday, following months of negotiations.

The two sides met again on Monday to continue negotiations, but failed to reach a settlement, meaning the strike will extend into at least a second day.

"This is not a strike I wanted," the mayor said. "It's unnecessary, it's avoidable and it's wrong."

Safety concerns

School officials said they had doubled the pay rise offered, for a total of 16% over four years.

We must do things differently in this city if we are to provide our students with the education they so rightfully deserve Karen Lewis, Chicago Teachers Union

"This is about as much as we can do," Chicago school board chief David Vitale told the Chicago Tribune. "There is only so much money in the system."

Karen Lewis, president of the Chicago Teachers Union, said the two sides were close to agreement on pay issues, but remained far apart on healthcare insurance provisions, job security and teacher evaluations.

"This is a difficult decision and one we hoped we could have avoided," Ms Lewis said.

"We must do things differently in this city if we are to provide our students with the education they so rightfully deserve."

Teachers arriving at picket lines on Monday morning, the second week of school for most American children, appeared defiant and ready for a battle.

Linda Kim, an English teacher striking outside Lake View High School, told the Tribune: "They said yesterday the strike is our choice.

"It could have been their choice to resolve this a while ago… They could resolve this any day, but they're choosing not to."

Officials said about 140 schools opened between 08:30 and 12:30 in order to provide breakfast and lunch to those children on free school meals.

Chicago's transit officials were offering free rides on buses and trains to students until the strike was resolved.

But there were concerns that having thousands of unsupervised children out of school could prompt violence and disorder, especially in areas of the city known for gang issues.

"What are these families going to do? Are you going to stay home from work today because of this?" Illinois Senator Dick Durbin, a Democrat, said on CNN on Monday morning.

Troubled history

Chicago's public schools have a troubled history, with high numbers of children coming from low-income families and many performing poorly in standardised testing.

Mr Emanuel cancelled a 4% pay rise last year, citing budget concerns, and asked the teacher's union to reopen their contract.

The mayor offered a 2% rise per year for four years in exchange for lengthening the school day by 90 minutes.

After the union refused, his attempts to negotiate on the longer school day with individual Chicago schools were blocked by the Illinois Educational Labor Relations Board.

Among other issues of concern, says the union, is a new evaluation test for teachers.

Union officials believe more than a quarter of Chicago's public school teachers could lose their jobs and benefits if their performance is evaluated on test scores, but the school district disputed that number.

The union president said the standardised tests do not take into account the effects of poverty and violence on students.

"Evaluate us on what we do, not the lives of our children we do not control," she added, upon announcing the strike.

Mr Emanuel, a former White House chief of staff for President Barack Obama, is attempting to revitalise city services and has backed the school reforms.

His proposals have been endorsed by the Obama administration's education secretary, Arne Duncan.

Mr Emanuel faced a $700m (£438m) school budget shortfall when he took office last year, and has pushed for the longer school day among other changes. His own children attend private schools in the city.

Ahead of landing in Chicago to raise money for his presidential bid, Republican candidate Mitt Romney criticised the teacher's union in a statement, saying those on strike had turned their backs on students.

White House spokesman Jay Carney told reporters that the administration hoped that "both sides are able to come together to settle this quickly and in the best interests of Chicago's students".