More than 30,000 public school teachers in Los Angeles, the nation's second-largest school district, walked out of classrooms and formed picket lines on Monday in what their union president says is "the fight for the soul of public education."

United Teachers Los Angeles (UTLA) said negotiations stalled and that the union didn't receive a new contract proposal over the weekend, after a strike was delayed from Jan. 10 because of potential court intervention.

"So here we are on a rainy day in the richest country in the world, in the richest state in the country, in a state as blue as it can be, in a city rife with millionaires, where teachers have to go on strike to get the basics for our students," UTLA President Alex Caputo-Pearl told a group of boisterous educators outside John Marshall High School in the Los Feliz district of Los Angeles.

The teachers walked off the job at 7:30 a.m. PST.

A school bus arrives with students as teachers and their supporters picket outside John Marshall High School in Los Angeles, on the first day of the teachers' strike, Jan. 14, 2019.(Robyn Beck/AFP/Getty Images) A school bus arrives with students as teachers and their supporters picket outside John Marshall High School in Los Angeles, on the first day of the teachers' strike, Jan. 14, 2019.

"Here we are in a fight for the soul of public education," Caputo-Pearl, a Los Angeles teacher for 22 years, said at a rally to kick off the strike. "The question is, do we starve our public neighborhood schools so that they are cut and privatized or do we reinvest in our public neighborhood schools for our students and for a thriving city? We are here to say from the picket lines of Los Angeles that we choose reinvestment."

Schools won't close, district schools superintendent Austin Beutner tweeted on Monday. The district hired hundreds of substitutes to cover for those on the picket lines demanding higher salaries and lower class sizes in a district that includes about 640,000 students.

The striking educators said they want to remedy classes that often have 40-plus students, a lack of counselors and nurses to support students, closed libraries and crumbling classrooms.

Union members told ABC News they'll strike "for as long as it takes."

Hiring substitute teachers is seen as "irresponsible" by the union, which asked parents to keep students home or join in the protest.

Teachers and their supporters picket outside John Marshall High School in Los Angeles on the first day of the teachers' strike, Jan. 14, 2019.(Robyn Beck/AFP/Getty Images) Teachers and their supporters picket outside John Marshall High School in Los Angeles on the first day of the teachers' strike, Jan. 14, 2019.

The latest offer from the district, proposed on Friday, includes adding about 1,200 counselors, nurses and librarians, as well as capping class sizes at 32 to 39 students, depending on the course.

The district also offered a 6 percent pay raise over the first two years of a three-year contract. The teachers have asked for 6.5 percent, retroactive to the 2017 fiscal year, and said the district's insistence on some proposals expiring after a year is disrespectful.

"We are extremely disappointed that United Teachers Los Angeles has rejected Los Angeles Unified’s revised offer without proposing any counteroffer. UTLA has refused to continue contract negotiations," district officials said in a statement.

But Arlene Inouye, UTLA's secretary, told ABC News, "We have not had an honest bargaining partner."

Victoria Costas, a mother and teacher, said there are 46 students in her daughter's math class.

"Classes are far too large to ever get individualized attention," she said.

Costas said she planned to join the teachers on the picket line "fighting with my daughter."

United Teachers Los Angeles (UTLA) union President Alex Caputo-Pearl (R) speaks to striking teachers and their supporters outside John Marshall High School in Los Angeles on the first day of the teachers' strike, Jan. 14, 2019.(Robyn Beck/AFP/Getty Images) United Teachers Los Angeles (UTLA) union President Alex Caputo-Pearl (R) speaks to striking teachers and their supporters outside John Marshall High School in Los Angeles on the first day of the teachers' strike, Jan. 14, 2019.

Another teacher, Erika Jones, said that 85 percent of the district's schools don't have a full-time nurse.

A tweet from the district's account said a work stoppage would "harm the students, families and communities we serve."

Beutner, a former investment banker and deputy mayor of Los Angeles, had no experience in education before taking over as superintendent last year.

"He's deeply out of touch with the needs of students and community," Caputo-Pearl, UTLA's president, said. "We are more convinced than ever that the district won't move unless there is a strike. We are going on strike for students, who we miss in our hearts right now. They deserve better."

Inouye, the union secretary, said in order to reach any kind of common ground with district leadership, "We need a place to start from."

Union supporters said they expect most of its approximately 35,000 members to join in the strike.

Last year, thousands of teachers in states including Arizona, Colorado and Oklahoma walked out of classrooms seeking higher pay and more resources for children.

"This is a fight worth the sacrifice," Inouye said.

ABC News' Marilyn Hicks and Leah LaRosa contributed to this report.