Construction is scheduled to begin Tuesday near San Diego on prototypes for a new barrier along the U.S. border with Mexico—the most visible step so far toward President Donald Trump’s long-promised border wall.

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The Trump administration last month announced four bidders had been selected to build concrete prototypes of border walls that will be 30 feet long and up to 30 feet high.

The sample walls will be built to scale, and function as usable barriers along the border. Federal officials will use the prototypes to help make final decisions in hiring wall builders.

Though the process is moving forward, there are many obstacles to the Trump administration’s plans for a wall. Environmentalists and the state of California have filed suits attempting to stop the wall; Congress hasn’t yet approved Mr. Trump’s request for the first $1.6 billion down payment.

About 650 miles of the roughly 2,000-mile U.S. border with Mexico already has some type of barrier—ranging from heavily fortified multilayered fencing to old steel landing mats used during the Vietnam War era. The Trump administration wants to both replace some existing barriers and add to others to create more substantial structures

Contract notices made public earlier this year called for bids for a solid wall that couldn’t be easily breached or scaled by illegal border crossers. The notices also called for wall proposals that would be aesthetically pleasing for people on the north side of the barrier. A second set of four bids for a wall made of “other materials” than concrete was awarded earlier this month.

Homeland Security officials have advised local, state and federal enforcement officials to be prepared for possible protests near the site of the prototype construction near the Otay Mesa border crossing.

U.S. Customs and Border Protection said in a memo dated Sept. 6 that large-scale protests could take place and “may arise with little or no warning.” The memo also warned that such protests could become violent. The memo was read to The Wall Street Journal by one law-enforcement official and confirmed by a second.

Mr. Trump made building a wall at the Mexican border a pillar of his presidential campaign, announcing the plan during a speech declaring his run for the White House in June 2015. That promise, and assurances that Mexico would pay for the multibillion-dollar project, became a rallying cry at Mr. Trump’s campaign events. Mexico has repeatedly denied that it would foot the bill.

The wall samples being built now are using money that was already in Homeland Security coffers. The government has said the concrete prototypes will cost an average of about $450,000.

U.S. Customs and Border Protection, the agency overseeing the project, hasn’t said how much the other prototypes will cost. Those sample walls are expected to be between 18 and 30 feet high.