Mayor Bill de Blasio on Thursday is marking the launch of his prekindergarten initiative -- an increase of the city's early education seats by 150 percent -- with a five-borough victory tour of programs. The expansion is meant to "shake the foundations of the school system and change it for the long term," he said at Inner Force Tots Early Education Center in East New York, Brooklyn.

"This is the vision that is coming to fruition today," de Blasio said. The reverberations will be felt for "years and years and years," he said.

The mayor said he remembers that his speech in October 2012 proposing universal pre-K was met with a "respectful air of incredulity."

"Today marks such a huge victory," first lady Chirlane McCray said.

More than 51,000 pre-K students began school Thursday, compared with 20,000 last year. The expansion was paid for with $300 million in state funds and executed in six months. The city seeks to expand enrollment to 70,000 next fall.

"When people say it can't be done, we're out to prove them wrong," said Schools Chancellor Carmen Fariña, applying what she said was a teacher's attitude to the realization of the mayor's pre-K vision. "New York City leads the nation and the world in what is a historic moment."

De Blasio, McCray and Fariña were joined by Brooklyn Borough President Eric Adams, United Federation of Teachers President Michael Mulgrew, City Council Speaker Melissa Mark-Viverito and other city leaders in chatting and playing with young students. Adams sat down at a miniature table with the children, and de Blasio later joked Adams is a budding teacher.

Parents dropping off their children at Inner Force Tots expressed gratitude for the expanded opportunities. The community-based organization has 145 pre-K students compared to 108 last year.

"Everything starts with a foundation," said Arnold Mbongo, 35, of East New York. His daughter Binta, 2, wearing a uniform jumper, waved happily on her way into the school. She can count to 25 and read a clock, Mbongo said.

The mayor emphasized that the city holds its pre-K programs to the "highest standards" not just academically, but on health and safety. The city canceled or delayed at least 45 of 1,700 pre-K sites' openings because of too-slow construction, health violations and bungled paperwork, among other problems.

The city is hiring an independent research firm and commissioning a $2 million study to assess the quality of its pre-K initiative.