More than 30,000 Los Angeles teachers demanding higher pay walked off the job in the second-largest US school system on Monday, union officials said.

About 640,000 students arriving on Monday for classes at some 900 campuses in the Los Angeles County School District were met by teachers carrying picket signs and rallying in the rain.

The union wants a 6.5 percent pay rise, more librarians, counsellors and nurses on campuses, smaller class sizes and less testing, as well as a moratorium on new charter schools.

Weeks of ultimately failed negotiations led to the decision to the launch the district's first teachers' strike in 30 years.

"It's an existential battle for the future of public education," teacher Mike Finn told Reuters news outside John Marshall High School in the Los Angeles neighbourhood of Los Feliz.

Thousands of teachers and supporters gathered outside City Hall, carrying umbrellas in the rain and chanting as they prepared to march toward district headquarters.

First day of our historic strike! On the line for our students. So much at stake. #UTLAStrong #WeAreLA #Strike4Ed pic.twitter.com/KkQ18zo0Pa — United Teachers Los Angeles (@UTLAnow) January 14, 2019

No formal talks were held over the weekend, dashing hopes of an 11th-hour deal between the teachers union and the school district.

Talks broke down on Friday when union bargaining chair Arlene Inouye said she and her fellow negotiators were "insulted" by the latest contract offer from district officials.

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Some teachers in Denver also walked out on Monday amid salary negotiations, according to a video posted on the Denver Classroom Teachers Association's Twitter page.

But the strike by United Teachers Los Angeles (UTLA) marks one of the largest and one of the few targeting a Democratic-controlled government.

Los Angeles County officials contend the strikers' demands are unaffordable and that a walkout hurts only students and parents.

Los Angeles Mayor Eric Garcetti, a Democrat who has said he is considering a run for his party's nomination in the 2020 presidential race, has largely avoided taking sides in the labor dispute.

In a video posted to Twitter on Sunday evening, Garcetti urged the district and union to reach an agreement as soon as possible. In a separate tweet, he said the city was offering free rides to students on transit systems.

'Set us up for failure'

Videos posted on Twitter by the UTLA, showed teachers and others marching with picket signs outside of a local public school chanting.

"You can't put students first if you put teachers last," the UTLA union posted on Twitter late on Sunday.

"When you cut our schools to the bone, you set us up for failure," UTLA wrote on Twitter on Monday. "When you reinvest in our schools, we thrive."

The strike has gained nationwide backing, including support from several unions, former teachers and labour rights advocates, the Los Angeles Times reported.

A handful of celebrities and the Screen Actors Guild‐American Federation of Television and Radio Artists have also proclaimed their support for the historic strike.

"We stand with our sisters and brothers from @UTLAnow as they fight for their students and schools," the Maryland State Education Association wrote on Twitter.

Classes for elementary, middle and high schools will continue through the strike, local media reported. Hundreds of substitute teachers have been hired and administrators have been reassigned.

County officials have said UTLA's demands would bankrupt the district.

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"Our commitment to our families is to make sure all of the money we have is being spent in schools. We are doing that," Beutner said in a statement on Friday.

"We hope UTLA leadership will reconsider its demands, which it knows Los Angeles Unified cannot meet."

The strike follows a number of similar actions across the country last year in which teachers in West Virginia, Kentucky and Oklahoma, among other states, launched strikes, walkouts and/or "sickouts" to call for education reform and better pay for teachers.

Nearly a year ago, West Virginia teachers picketed for more than a week before lawmakers officials approved a five percent pay raise for all state workers.

In April 2018, Oklahoma teachers ended a nearly two-week strike after securing pay raises and increased education spending. In Kentucky and Arizona, teachers won a boost in funding and wages last year after walkouts.