Visitors tour the new African Burial Ground National Monument in New York. A dedication ceremony Friday marked the formal opening of the memorial constructed to honor the memories of the estimated 15,000 Africans buried at the original seven-acre site after their discovery in 1991.

New York ? It was a day that had been a long time coming, and for the community leaders who gathered Friday to dedicate a memorial at the once-forgotten grave site of thousands of African slaves, it was also a day of regret.

“Forgive us for disregarding your precious gifts to this world,” the Rev. James A. Forbes Jr. said to the long-dead slaves and free blacks who were interred beneath lower Manhattan, then forgotten for decades as the city sprouted skyward above their remains.

Many speakers at the African Burial Ground memorial lamented the lack of recognition those buried there had experienced in life and after death, and vowed to make the memorial a permanent reminder of their sacrifices.

Sixteen years after the remains were rediscovered, onlookers lined two city blocks Friday for a chance to file through “The Door of Return.” The entryway is named in contrast to the door of no return, the title once given to departure points where slaves were stolen away from their African homelands.

Walking through the narrow, reflective-granite structure, the memorial’s first visitors stepped out into a sunken court engraved with a map of the lands and waters that once supported the slave trade, as well as the identifying details of some of the women, men and children buried nearby.

“Bid ’em in,” poet Maya Angelou sang to the crowd assembled for the dedication, telling the story of the auctioning of a young girl, stripped nude on the block.

“All of us are being ‘bidded in’ according to how we forget you,” she said to those interred beneath seven grassy mounds alongside the memorial. “And we will not forget you.”

Mayor Michael Bloomberg told those assembled that forgetting might sometimes seem the easier route. New York City thrived during the slave trade and much of the early metropolis was built with slave labor.

The construction workers who stumbled across the site “brought to light one of the most uncomfortable and tragic truths in the history of our city,” Bloomberg said. “Part of atoning for such a terrible injustice is to acknowledge it.”

The Manhattan site was declared a national monument last year. At the dedication ceremony, Lt. Gov. David Paterson decried the lack of a national memorial in Washington, D.C., to mark the sacrifices slaves made throughout the nation.

About half the skeletons unearthed at the site were those of children younger than 12. The vast majority died as a result of violence, Paterson said.

Performances by dancers in slavery-era costume and drummers in traditional garb punctuated the ceremony. Actor Avery Brooks performed a spoken-word piece, and the trio Three Mo’ Tenors gave a sorrowful rendition of “Make Them Hear You,” from the musical “Ragtime.” Actor Sidney Poitier joined in a processional through the site.

It wasn’t always clear whether the grave site, much of which is undisturbed beneath several square blocks of office towers, would be marked. When the skeletons were discovered, protesters called on the government to stop construction of an office building planned for the spot.

After archeologists unearthed the bones of 419 individuals, there were numerous delays as the remains were studied and researchers and the government fought over funding. Eventually, the bones were placed in hand-carved caskets and buried in crypts alongside what is now the memorial. The project cost more than $50 million.