The first toddlers have been accepted into New York City’s free public “3-k” preschool program, Mayor Bill de Blasio said Thursday. It’s being inaugurated in some of the city’s poorest neighborhoods.

Nearly 800 students who are 3 or about to turn 3 will begin “3-k” classes in September in two districts that cover the South Bronx and Brooklyn’s Brownsville, Ocean Hill and East New York neighborhoods, de Blasio said.

“We’re literally at the beginning of a new era in our education of our children,” de Blasio said at a news conference about the program. “We can’t lose this opportunity to shape these extraordinary young minds while the chance is there, while they’re most open to growth.”

The city Department of Education predicts the program will cover about 62,000 3-year-olds by the fall of 2021.

The curriculum for the classes — which will run 6 hours and 20 minutes 5 days a week — will cover learning new words, playtime, science, arts and social-skills development.

There were about 2,300 applicants to the program, with a preference given to families who live in the district.

Guaranteeing universal prekindergarten classes to every 4-year-old was a cornerstone of de Blasio’s winning 2013 mayoral campaign, and he’s been credited with delivering on the promise.

But there is controversy involving state lawmakers, providers and other critics over whether de Blasio can achieve a similar result for every 3-year-old. The program will require hundreds of classrooms, hiring of staff and hundreds of millions in state and federal funding.

A spokeswoman for the mayor told The New York Times earlier this year that there were “still important program development discussions to be had, but the mayor refuses to wait.”