Construction workers applauded as the upright spire began heading skyward at midday Thursday on its way to the roof.
The spire's temporary home will be a platform at the top of the trade center. When it's fully installed, at a later date, it will soar to a symbolic 1,776 feet.
The 408-foot spire will be used as a world-class broadcast antenna.
The new tower is at the northwest corner of the trade center site. The area is well on its way to reconstruction with the 72-story Four World Trade Center and other buildings.
The raising of the pieces was originally scheduled for Monday, but was postponed because of weather issues.
Installation of the 800-ton, 408-foot spire began in December, after 18 pieces were shipped from Canada and New Jersey.
With the beacon at its peak to ward off aircraft, the spire will provide public transmission services for television and radio broadcast channels that were destroyed on Sept. 11, 2001, along with the trade center towers.
Overlooking the Statue of Liberty and Ellis Island, the high-rise is scheduled to open for business in 2014.
Monday's postponed celebration of the reconstructed trade center was to come days after a reminder of the terror attack that took nearly 3,000 lives: the discovery of a rusted airplane part wedged between a nearby mosque and an apartment building - believed to be from one of the hijacked planes that ravaged lower Manhattan.
The new tower's crowning spire is a joint venture between the ADF Group Inc. engineering firm in Terrebonne, Quebec, and New York-based DCM Erectors Inc., a steel contractor.
The tallest building in the Western Hemisphere is the Willis Tower in Chicago. The world's tallest building, topping 2,700 feet, is in Dubai.