Last Updated 5:07 p.m. ET
(CBS/AP) CHICAGO - Thousands of teachers, parents and supporters marched through downtown Chicago on the first day of a school strike.
The crowd Monday afternoon stretched for several blocks and was expected to swell through the early evening and into the city's rush hour. Some protesters carried signs that said "Chicago Teachers United" and "Fair Contract Now." Others waved red pom-poms and chanted. Earlier in the day, thousands of teachers picketed around neighborhood schools.
The city's 25,000 public school teachers are on strike for the first time in a quarter-century, after the latest contract talks broke down Sunday with no deal to avert a walkout.
Chicago Teachers Union President Karen Lewis said late Sunday night there had been some progress in contract talks, but "we have failed to reach an agreement that will prevent a labor strike."
The city's public school teachers make an average of $71,000 a year. Both sides said they were close to an agreement on wages. What apparently remains are issues involving teacher performance and accountability, which the union saw as a threat to job security.
Late Sunday, Mayor Rahm Emanuel, who has already forced teachers to lengthen their school days, said he was "disappointed" in the union's decision to continue with a strike.
"I am disappointed that we have come to this point, given that even all the other parties acknowledge how close we are because this is a strike of choice," Emanuel said. "Because of how close we are, it is a strike that is unnecessary."
On Monday, the mayor urged negotiators to come up with a new contract for the city's teachers quickly for the sake of the children.
Also expressing disappointment: Republican presidential candidate Mitt Romney, who released a statement in advance of a visit to Chicago for fundraisers. It said in part, "I am disappointed by the decision of the Chicago Teachers Union to turn its back on not only a city negotiating in good faith but also the hundreds of thousands of children relying on the city's public schools to provide them a safe place to receive a strong education."
Romney then accused President Obama of siding with the teachers union.
After talks ended last night, Chicago Board of Education President David Vitale said he believes CPS officials made their best possible offer to teachers.
"There's only so much money in the system. There's only so many things that we can do that are available to us," Vitale said. "At this juncture, it is clearly their decision. ... We've done everything we can."
Lewis said the two sides were close to agreement on a contract, but not close enough.
"We are not far apart on compensation, however we are apart on benefits," Lewis said. "We want to maintain the existing health benefits."
Lewis said the union is also concerned that a proposed new teacher evaluation system "could result in almost 6,000 teachers - or nearly 30 percent of our membership - being discharged within one or two years. This is unacceptable and leads to instability for our students."
She said the new evaluation system would rely too much on students' standardized test scores.
"This is no way to measure teacher effectiveness at all," she said.
A dispute involving public sector employees in Chicago was somewhat surprising, said CBS News correspondent Dean Reynolds, given the generous packages unions here have won in the past. In addition, a teacher strike in the hometown of a president who stresses the importance of education could also be seen as something of a political embarrassment.
The union had set a midnight deadline for a walkout. Negotiations are to resume on Monday at 10 a.m. at a secret location.
Teachers began to walk picket lines at schools shortly before 6:30 a.m., and students and parents were left looking for alternatives.
Chicago Public Schools is opening about 140 schools for children from 8:30 a.m. to 12:30 pm.; however, no instruction will take place.
"They're going to lose learning time," said Beatriz Fierro, whose daughter is in the fifth grade on the city's Southwest Side. "And if the whole afternoon they're going to be free, it's bad. Of course you're worried."
About 45,000 students who attend the city's charter schools will not be affected by the walkout - their schools will stay open.
In addition, churches, libraries and community organizations will be providing students with activities.
Before the strike, some parents said they would not drop their children at strange schools where they didn't know the other students or supervising adults. On Monday, as only a trickle of students arrived at some schools, April Logan said she wouldn't leave her daughter with an adult she didn't know. Her daughter, Ashanti, started school just a week earlier.
"I don't understand this, my baby just got into school," Logan said at Benjamin Mays Academy on the city's South Side before turning around and taking her daughter home.