DENVER (AP) — Striking teachers on Monday picketed outside of schools and marched through Denver's streets as car horns blared in support of the latest U.S. walkout amid a swell of educator activism that has affected at least a half-dozen state over the last year.

Just over half of the 4,725 teachers working in district-run schools called in absent for Denver's first strike in 25 years. Some students crossed picket lines to get to class as schools remained open with administrators and substitute teachers in classrooms.

In one school, students danced and chanted in the hallways as they walked out to demonstrate to support their teachers. Other students joined hundreds of teachers and union members in a march past City Hall that held up traffic in downtown Denver.

Science teacher Abraham Cespedes said Denver educators were empowered by recent teacher activism and strikes from Arizona to West Virginia.

"By us doing this we finally became united," he said, marching with fellow teachers, members of other unions and students.

The strike affecting about 71,000 students in the school district comes about a year after West Virginia teachers launched the national "Red4Ed" movement with a nine-day strike in which they won 5 percent pay raises.

There have since been walkouts in Washington state, Arizona, Kentucky and Oklahoma, as teachers protest low pay, crowded classrooms and staffing shortages.

Most recently, Los Angeles teachers went on strike last month. That walkout ended when teachers received a 6 percent raise and promises of smaller class sizes and the addition of more nurses and counselors.

In Denver, the dispute is over the school district's incentive-based pay system. The city's school district gives bonuses ranging from $1,500 to $3,000 a year to teachers who work in schools with students from low-income families, in schools that are designated high priority or in positions that are considered hard to staff, such as special education or speech language pathology.

The union is pushing to lower or eliminate some of those bonuses to free up more money that would be added to overall teacher pay.

The district sees the disputed bonuses as key to boosting the academic performance of poor and minority students. Denver teachers say the reliance on bonuses in the district leads to high turnover, which they say hurts students, and that spending money on smaller class sizes and adding support staff, like counselors, is the best way to help disadvantaged students learn.

Some teachers have become ineligible for bonuses after their schools lost their official low-income status as parts of the city undergo gentrification.

The district has proposed raising starting teacher pay from $43,255 to $45,500 a year. That's $300 a year less than the union's proposal, which would add $50 million a year to teacher base pay, according to union officials.

At a news conference, district Superintendent Susana Cordova said negotiations will resume on Tuesday.

"It is a problem for our kids not to have their teachers in class," Cordova said. "I want to get this done now. So I'm very happy that we will be back at the table."

State House Majority Leader Alec Garnett, a Democrat, said the strike underscores the need for lawmakers to fix conflicting laws that restrict state public school spending by hundreds of millions of dollars a year.

"There is no quick fix to the school finance issues that the state is facing. And so if this is a way that helps bring people to the table to solve it, then that's a good thing," Garnett said.

The state says a walkout will cost about $400,000 a day and would consume 1 to 2 percent of the district's annual operating budget in about a week.

In encouraging both sides to come to an agreement, Gov. Jared Polis has pointed out that this money will no longer be available to help pay teachers if it is spent on the strike.

The strike happened after the administration of Polis, a Democrat, decided last week not to get involved, believing the negotiating positions of administrators and teachers were not very far apart.

However, Polis said the state could decide to intervene and suspend the strike for up to 180 days if the walkout drags on.

The state does not have the power to impose any deal on either side. But it can try to help the union and school district reach a deal and can require them to participate in a fact-finding process.