LOS ANGELES—The U.S. Department of Homeland Security is worried that planned construction of short stretches of border wall prototypes near San Diego could spark large-scale protests and violence, according to an intelligence alert described to The Wall Street Journal by two law-enforcement officials.

U.S. Customs and Border Protection warned state and local law enforcement that protests “may arise with little or no warning” and could pose a “threat to U.S. personnel and infrastructure” along the border, according to the three-page memo dated Sept. 6.

The agency advised that protesters may try to block construction and U.S. Border Patrol vehicles along undeveloped stretches of land near the edge of a secondary fence near San Diego.

Construction on wall prototypes is expected to begin Sept. 26, according to the alert. It was unclassified but marked as law-enforcement sensitive, one of the officials said.

The alert didn’t detail government plans to deal with any such protests.