The union immediately condemned the move as an act of vindictiveness by a "bullying" mayor.
Emanuel spokeswoman Sarah Hamilton said city attorneys filed a request in the Cook County Circuit Court to force Chicago Teachers Union members off the picket line and back into classrooms.
The request argues that the labor action is illegal because state law bars the union from striking on anything but economic issues, and that the work stoppage is focused instead on such issues as evaluations, layoffs and recall rights.
The 700-page filing also contends the strike presents a danger to public health and safety, partly because more than 80 percent of 350,000 public students rely on school meals for their basic nutrition; it says 50,000 others, including autistic students, depend on special instruction. And out of school, children are more prone to fall victim to violence, it says.
"At a critical time in their lives, a vulnerable population has been cast adrift by the CTU's decision to close down the schools, with consequent grave implications for the residents of the city of Chicago," the court document says.
The union blasted the city's decision to resort to legal action.
In a statement released later Monday, the CTU said the filing appeared to be "a vindictive act."
"This attempt to thwart our democratic process is consistent with Mayor Emanuel's bullying behavior toward public school educators," the union said in the statement.
A spokesman for the city's law office, Roderick Drew, arguments have not been scheduled and that he didn't expect a ruling until later in the week.
"Nothing will be set today," he said. "Beyond that, we don't know."
The union and school leaders seemed headed toward a resolution at the end of last week, saying they were optimistic that students in the nation's third-largest school district would be back in class by Monday. But teachers uncomfortable with a tentative contract offer decided Sunday to remain on strike, saying they needed more time to review a complicated proposal.
That's when Emanuel said he would take legal action.
"This was a strike of choice and is now a delay of choice that is wrong for our children," Emanuel said in a written statement Sunday night.
The strike is the first for the city's teachers in 25 years and has kept students out of class, leaving parents to make other plans.
Working mom Dequita Wade said that when the strike started, she sent her son 15 miles away to a cousin's house so he wouldn't be left unsupervised in a neighborhood known for violent crime and gangs. She was hoping the union and district would work things out quickly.
"You had a whole week. This is beginning to be ridiculous," Wade said. "Are they going to keep prolonging things?"
Months of contract negotiations have come down to two main issues: teacher evaluations and job security.
Union delegates said they felt uncomfortable approving the contract because they had seen it only in bits. The union will meet again Tuesday, after Rosh Hashana, the Jewish new year holiday.
"There's no trust for our members of the board," Chicago Teachers Union president Karen Lewis told reporters Sunday night. "They're not happy with the agreement. They'd like it to actually be a lot better."
The strike has shined a spotlight on Emanuel's leadership, and some experts suggest the new contract - which features annual pay raises and other benefits - is a win for union.
"I'm hard-pressed to imagine how they could have done much better," said Robert Bruno, a professor of labor and employment relations at the University of Illinois at Chicago. "This is a very impressive outcome for the teachers."
With an average salary of $76,000, Chicago teachers are among the highest-paid in the nation, and the contract outline calls for annual raises. But some teachers are upset it did not restore a 4 percent raise Emanuel rescinded last year.
Emanuel pushed for a contract that includes ratcheting up the percentage of evaluations based on student performance, to 35 percent within four years. The union contends that does not take into account outside factors that affect student performance, such as poverty and violence.
The union pushed for a policy to give laid-off teachers first dibs on open jobs anywhere in the district, but the city said that would keep principals from hiring the teachers they think are most qualified.
The union has engaged in something of a publicity campaign, telling parents about problems that include a lack of important books and basic supplies.
Some parents said they remain sympathetic to teachers.
"I don't think they're wrong. The things they're asking for are within reason," said Pamela Edwards, who has sent her 16-year-old daughter to one of about 140 schools the district has kept open during the strike to provide meals and supervision.
Others said they understand why teachers are taking their time.
"What's the point of going on strike if you don't get everything you need out of it?" said Becky Malone, mother of a second grader and fourth grader, who've been studying at home and going to museums over the last week. "For parents, it'll be no more of a challenge than it's been in the past week."