Los Angeles teachers went on their first strike in three decades Monday, with an estimated 30,000 walking out following a breakdown in negotiations between the their union and the LA Unified School District.

The union, United Teachers Los Angeles, is demanding an immediate 6.5 percent pay raise, made retroactive for one year, as well as smaller class sizes and more support staff hiring. The city counter-offered a 6 percent raise, phased in over two years, an offer the union termed an insult.

"UTLA educators, #LAUSD parents, and students on the picket line this morning in the rain demanding smaller class sizes and justice," the union tweeted.

L.A. teachers are already among the highest paid in the nation, getting more than $78,000 annually, according to the Los Angeles Times. Their pay is still below salaries in some of the wealthier suburbs surrounding the city, however.

The city reported that all elementary, middle and high schools nevertheless remained open, the city having authorized $3 million for replacement hires should talks sour. An estimated 600,000 students participate in the school system.

"We are extremely disappointed that United Teachers Los Angeles (UTLA) has rejected Los Angeles Unified’s revised offer without proposing any counter offer. UTLA has refused to continue contract negotiations," the district says on its website.

[Opinion: Why the Los Angeles teachers union must be broken]