More than 500,000 children were shut out of lessons Tuesday as school workers in Los Angeles began a three-day walkout over pay.

Bus drivers, cafeteria workers and special education assistants employed by Los Angeles Unified School District -- the second biggest in the United States -- are striking over what they say are "poverty wages."

The roughly 30,000 members of the powerful United Teachers Los Angeles union will not cross the picket line and will therefore not be working, the union said.

Service Employees International Union Local 99 says many of its members earn as little as $25,000 per year.

"Caring for students is one of the most meaningful careers," tweeted US Congressman Adam Schiff, from southern California.

"I stand with the bus drivers, custodians, teachers' aides and others... No one can live on $25,000. We must invest in schools and pay fair wages."

Los Angeles is one of America's most pricey places to live, with high taxes and expensive grocery and utility bills.

Median rent in the city is $3,000 per month, according to property website Zillow.

LAUSD officials said they have offered a 23 percent increase, along with a three percent "cash-in-hand bonus."

The union is demanding a 30 percent pay raise, with an additional boost for the lowest-paid workers.

Los Angeles Mayor Karen Bass said the city was stepping in to provide food and shelter to vulnerable children who would normally get such services at school.

"With more than 500,000 students and their families affected by today's school closures, the City is providing meals and safe places for young people at dozens of locations across Los Angeles," she said.

"I will make sure the wellbeing of LA students always comes first as I continue to work with all parties to reach an agreement to reopen the schools and guarantee fair treatment of all LAUSD workers."

The dispute comes as Americans are battered by a persistent cost of living crisis, with prices of everyday goods and services rocketing up over the last year or so.

Figures released last week showed the consumer price index rose six percent from a year ago.