Chicago teachers hold placards as they walk the picket line outside the headquarters of Chicago Public Schools in Chicago.

Chicago - Thousands of public school teachers formed picket lines in Chicago on Monday and parents scrambled for child care during teachers' first strike in a quarter century over reforms sought by Mayor Rahm Emanuel and endorsed by President Barack Obama's administration.

Some 29 000 teachers and support staff in the nation's third-largest school system were involved, leaving parents of 350 000 students between kindergarten and high school age to find alternative supervision.

“There's no excuse for either side for not coming to an agreement,” said Faith Griggs-York, mother of a first-grader at Agassiz Elementary School, as she dropped her daughter off at a community centre a few kilometres from the school.

“I think both sides, because of what they are doing to parents and because of what they are doing to kids, should be embarrassed,” Griggs-York said.

The teachers' union called the strike on Sunday night after months of negotiations did not resolve major disagreement over public education reforms. Talks resumed between the union and school district on Monday.

Emanuel is among a number of big city US mayors who have championed school reforms and Obama's education secretary, Arne Duncan - a former head of Chicago public schools - has endorsed them.

The school district's charter schools, which account for about 12 percent of students, opened as usual. The mayor wants to expand the number of charter schools, which are publicly funded but non-union.

Churches, community centres, some schools and other public facilities opened to care for thousands of children under a $25-million strike contingency plan financed by the school district. The children were supervised half a day and received breakfast and lunch, allowing some parents to work.

The union has called the plan to care for children during the strike a “train wreck”. It warned that caregivers for the children do not have proper training, and there are fears of an increase in gang-related violence in some high-crime areas.

About 20 teachers picketed in front of Overton Elementary School on Chicago's South Side, wearing red T-shirts, carrying strike signs and singing “We're not going to take it”, the chorus from the rock band Twisted Sister's popular anthem.

Several passing cars honked in support, prompting loud cheers from the striking teachers.

Chicago's South Side, often mentioned by First Lady Michelle Obama in reference to her humble roots, is one of the city's poorest districts and has a large African-American population.

Laura Gunderson, a fourth-grade teacher at Nettelhorst Elementary School, held a “proud union home” sign outside the North Side school with 55 other teachers, aides and clerks lining both sides of the street.

“My heart sank on Friday night when I clocked out and realised I was not going to be teaching Monday,” said Gunderson, a teacher for 26 years.

The Chicago confrontation also threatens to sour relations between Obama's Democratic Party and labor unions before the presidential election on November 6.

While Obama is expected to win the vote in Chicago and his home state of Illinois, union anger could spill into neighbouring Midwestern states such as Wisconsin, Michigan and Ohio, where the race with Republican challenger Mitt Romney is much closer.

White House spokesperson Jay Carney said Obama was aware of the situation in Chicago. “We hope both sides are able to come together to settle this quickly in the best interest of Chicago's students,” Carney said.

Campaigning in the battleground state of Ohio, Romney criticised the teachers union. “I want our kids to have the skills they need for the jobs of tomorrow and that means put our kids first and put the teachers' unions behind.”

Emanuel said two main issues remain in the dispute: his proposal that teachers be evaluated based in part on student performance on standardised tests, and more authority for school principals.

Union President Karen Lewis, who has sharply criticised Emanuel, said standardised tests do not take into account inner city poverty as well as hunger and violence in the streets.

More than 80 percent of Chicago students qualify for free lunches because they come from low-income households, and Chicago students have performed poorly compared with national averages on most reading, math and science tests.

Union officials said more than a quarter of Chicago public school teachers could lose their jobs if they are evaluated based on the tests.

“Evaluate us on what we do, not the lives of our children we do not control,” Lewis said in announcing the strike.

Dick Simpson, a former city council member, or alderman, said that past Chicago mayors would have called negotiators to the mayor's office to get a deal by offering the union concessions. But dire financial straits preclude Emanuel from throwing money at the problem. The last Chicago teachers strike in 1987 lasted 19 days.

“Most parents now are supporting the teachers. If the strike were to go on that long the public would be mad,” said Simpson, a political science professor at the University of Illinois at Chicago. - Reuters