Mayor Bill de Blasio Saturday dedicated a memorial to the untold number of human beings who were bought and sold centuries ago at a Wall Street slave market and helped build New York.

Speaking Saturday at Wall and Water streets with his wife, Chirlane McCray, at his side, de Blasio recalled the people who were forced to build what was then New Amsterdam and the beginnings of the burgeoning metropolis of New York City.

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"Today, we recall that history," de Blasio said Saturday afternoon. "It was true two, three centuries ago, even though it was never acknowledged. It was true then, it is true today. It will be true tomorrow. Black lives matter."

The plaque the mayor dedicated honored those who passed through a slave market, located near Pearl and Water streets, a market in use until at least 1762. In New York's earliest days, almost half of New York households had at least one slave, and by the 18th century, 1 in 5 New Yorkers was enslaved, according to the city Parks Department. Slaves were responsible for building much of early New York, including clearing forest for the construction of what would become Broadway and Wall Street.

"Today, we seek to uncover New York's hidden history," de Blasio said.

A new plaque is shown marking the site of a colonial-era Wall Street slave market at Wall Street and Pearl Street in Manhattan on Saturday, June 27, 2015. Photo Credit: Steven Sunshine

De Blasio said the legacy of slavery and racism -- "the stains on American history" -- continue today, drawing parallels to the racially motivated massacre this month of nine black people in Charleston, South Carolina, by a gunman at a church Bible study meeting.

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The mayor bowed his head for a moment of silence to honor the slain churchgoers.