Gospel singer BeBe Winans performed "God Bless America" as the beam and a U.S. flag were hoisted 977 feet by a crane and placed on top of the building. More than 100 construction workers signed their names to the steel at a ceremony that included elected officials and developer Larry Silverstein.
Sally Rexach, a nurse who aids workers at the site as she did during the rescue and recovery work following 9/11, had tears streaming down her face as she watched the beam rise into the sky.
"Ten years later, it's pretty remarkable. This is a very proud moment. It's full circle," she said.
John Rzeznik, a project manager at the site, spoke of his pride in the endeavor.
"Everybody's put their blood, sweat and tears into this," he said.
Speaking to the crowd, Silverstein said his aim and that of the construction workers had been "to give New Yorkers back the city terrorists tried to take away."
After years of funding disagreements that at times seemed to threaten progress at the site, Silverstein acknowledged, "It's been a very tough time." But, he said, "I've always believed in downtown New York."
The tower, scheduled to open late next year, is expected to be the first completed on the World Trade Center site since 9/11.
The building will primarily house commercial offices, and a third of the office space will be set aside for the headquarters of the Port Authority of New York and New Jersey, which owns the trade center site.