A chain cutting ceremony was held for the new $6.4 billion eastern span of the bridge. Lt. Gov. Gavin Newsom and other dignitaries were in attendance. After the chain cutting, came a procession of classic cars.
The entire bridge was shut down on Wednesday night as crews got to work. The new span replaces the old unsafe structure damaged in the 1989 Loma Prieta earthquake.
It is designed to endure 150 years and withstand the strongest earthquake estimated by seismologists to occur at the site over a 1,500-year period.
The bridge was originally scheduled to reopen at 5 a.m. Tuesday.
The Associated Press contributed to this report.