LOS ANGELES (Reuters) – Tens of thousands of striking Los Angeles teachers rallied near City Hall on Friday, joined by parents and students as contract talks the mayor called “productive” resumed behind closed doors after a marathon negotiating session the day before.

Some 30,000 teachers walked off the job on Monday in their first strike against the Los Angeles Unified School District in 30 years, demanding higher pay, smaller classes and more support staff. They also have called for curbs on the steady expansion of independently managed charter schools, arguing that they divert resources from traditional classroom instruction.

“We are willing to go as long as it takes and work as hard as we need to to get a fair contract,” union bargaining chair Arlene Inouye told supporters at City Hall, adding that talks were expected to last through the three-day holiday weekend.

The nation’s second-largest school district has kept its 1,200 campuses open with administrators and substitute teachers, but only a fraction of the 500,000 affected students have turned up, mostly gathered in gymnasiums and assembly halls.

At the urging of Mayor Eric Garcetti, negotiators for the United Teachers Los Angeles union and the district returned to the bargaining table on Thursday for the first time since talks broke off a week ago.

Garcetti, who lacks direct authority over the school district but is mediating the talks, said in a statement that the two sides had a “productive day of contract negotiations” on Thursday that ran past midnight.

Negotiators were back at it on Friday but offered no further clues about their progress after agreeing to a news blackout.

Seeking to strengthen its position at the bargaining table, the union staged a boisterous mass rally near City Hall, where tens of thousands of teachers, parents and students in red T-shirts filled a downtown park and surrounding streets.

“You can see how serious we are here. We’re not backing down,” said Richard Peterson, 65, a teacher at Northridge Middle School in the city’s San Fernando Valley.

The job action follows a wave of teacher strikes last year across the United States over salaries and school funding, including walkouts in West Virginia, Oklahoma and Arizona. But the Los Angeles work stoppage differs in terms of educators facing a predominantly Democratic political establishment largely sympathetic to their cause.

Several possible contenders for the 2020 Democratic presidential nomination, Garcetti among them, have voiced solidarity with the strike. Support also was running high among parents and the public at large as reflected in a recent survey of Los Angeles residents.

School Superintendent Austin Beutner has said the teachers’ demands, if fully met, would be too great a budget strain. Union President Alex Caputo-Pearl has said sufficient funding is available if the district had the right priorities.

The district said in a statement late Thursday the strike had already cost about $100 million – education funding in California is based on daily attendance – and that “our students are missing out on the opportunity to learn.”

Caputo-Pearl said Thursday that “key differences” remained between the two sides, with probably the biggest obstacle posed by the demand to hire enough teachers to substantially reduce class sizes. They have been without a contract for nearly a year.