All vehicle traffic heading into Mexico through the nation's busiest border crossing was blocked Friday by the California Highway Patrol ahead of expected demonstrations in Tijuana.
Four CHP vehicles stocked with riot gear were parked across empty southbound Interstate 5 at the San Ysidro Port of Entry, with no protesters immediately in sight.
Traffic heading south for the Labor Day weekend was being diverted to a nearby border checkpoint at Otay Mesa, about 10 miles east, according to CHP Officer Terry Brollini.
U.S. Customs and Border Protection officials have urged travelers to avoid the border crossings during the protests over Mexico's pension laws, citing Mexican news reports that traffic in both directions may be blocked for up to six hours at San Ysidro.
Pedestrians were unaffected by the blockage.
Traffic coming into the U.S. from Mexico was unaffected, according to CBP spokesman Vince Bond.
An average of about 30,000 pedestrians, 35,000 vehicles and 250 buses enter the U.S. each day at San Ysidro, making it the country's busiest crossing by far.
In June, about 10,000 demonstrators in Mexico briefly blocked traffic between San Diego and Tijuana.
The demonstrators, including many teachers, protested a Mexican law passed earlier this year that raises the retirement age for workers to 60 from 50 and introduces individual retirement savings accounts for government workers.