Dozens of truckers protested a program that will allow up to 100 Mexican trucking companies to freely haul their cargo anywhere in the United States.
The U.S. Transportation Department was expected to begin issuing operating permits in the pilot program as early as Thursday.
The program is designed to study whether opening the U.S.-Mexico border to all trucks can be done safely.
As of Thursday morning, 38 Mexican firms were poised for U.S. permits, said Melissa Mazzella DeLaney, a spokeswoman for the Federal Motor Carrier Safety Administration, which regulates truck safety.
The Teamsters union and Sierra Club oppose the program and sued to try to stop it, arguing that there would not be enough oversight of the drivers coming into the U.S. from Mexico and public safety would be endangered.
Government lawyers countered the program was a necessary part of the 1994 North American Free Trade Agreement and said trucks enrolled in the program would meet U.S. regulations.
A federal appeals court ruled Aug. 31 that the Bush administration could move ahead.
NAFTA requires that all roads in the United States, Mexico and Canada be opened to carriers from all three countries. Since 1982, Mexican trucks have been allowed to operate only within a 25-mile (40-kilometer) zone along the border. There, they transfer loads to U.S. trucks to go elsewhere in the country.
Dozens of truckers led by the Teamsters mixed with some anti-illegal immigration activists to protest near San Diego's Otay Mesa border crossing, some flashing signs that read, "NAFTA Kills" and "Save American Highways."
Business was uninterrupted, said Lt. Hector Paredes of the California Highway Patrol, which inspects about 3,000 trucks a day at the crossing.
"We're already inspecting Mexican trucks and will continue to inspect them the same way," Paredes said. "These trucks already haul product from Tijuana to San Diego. Now they will be able to go beyond San Diego."