SOME 20,000 AFRICANS buried in a long-hidden 18th century cemetery in lower Manhattan were given one of America's highest honors yesterday, as the White House designated the sprawling grave site a national monument.

The burial ground, a 5-acre area near City Hall, was uncovered more than 14 years ago when construction workers broke ground to build a new federal office tower at Duane and Elk Sts.

Rep. Charles Rangel (D-Harlem) said President Bush's signing of the presidential proclamation bestowing the rare honor is a painful milestone.

"I accept this with the same type of pain that a family of a murder victim would feel when the killer points out where he buried the bodies," Rangel said.

The burial ground contains the remains of free and enslaved Africans who worked as carpenters, barrel makers and dockworkers in the formative years of New York City, said historians.

The African Burial Ground, which lies roughly 20 feet below the surface, joins an elite roster of national monuments, such as the Statue of Liberty and Mount Rushmore, which have been awarded the rare presidential designation. The site has already been designated a national landmark.

The special recognition as a national monument means that the burial ground will preserve this chapter of history for all time, said officials.

Construction is scheduled to begin this year on a granite monument, designed by architect Rodney Leon, that will memorialize the dead.