Work began on Tuesday on the construction of prototypes for the wall along the US-Mexico border that US President Donald Trump wants to build to stop illegal immigration.

Customs and Border Protection said that activities commenced in Otay Mesa, California, where the prototypes will be erected in a zone that is carefully monitored by US security forces, which have developed a contingency plan if wall opponents decide to stage demonstrations at the site of the controversial project.

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The six companies selected in the bidding process will build four cement prototypes and four made of other materials and will have 30 days to erect their models of a portion of the prospective wall, efe reported.

The prototypes will be between 5.5 and 9.1 meters (18 and 30 feet) high and will be designed to dissuade undocumented migrants from crossing the border in the area and fulfil the requirements established by the Border Patrol, authorities said in a statement.

“We are committed to securing our border and that includes constructing border walls. Our multi-pronged strategy to ensure the safety and security of the American people includes barriers, infrastructure, technology and people,” Ronald Vitiello, acting deputy commissioner of the US Customs and Border Protection, said in a statement on Tuesday.

This part of San Diego, where a double fence already runs along the border, was selected because it is a zone with a high incidence of criminal activity.

The CBP chief said that “Moving forward with the prototypes enables us to continue to incorporate all the tools necessary to secure our border.”

The prototypes will be build one beside the other on a plot of ground just yards from the fence dividing Otay Mesa from Tijuana, Mexico.

At the work site, dozens of heavy vehicles and construction equipment have been allowed through the security cordon around the restricted area.

In nearby streets, signs have been posted prohibiting vehicles from parking there between September 26 and November 10.

Along most of the 3,180 km US-Mexico border the wall project includes clearing an area to allow US authorities to see what is happening on the Mexican side and, in some zones where the danger of illegal crossings is greater, a solid wall made of cement-like materials is planned.

Building such a wall was one of Trump’s key election promises and, because the billions of dollars it will cost have not yet been allocated by Congress, the mogul — who had assured the public that Mexico would actually pay for the wall — has threatened to engineer a partial government shutdown if Democratic lawmakers refuse to include the funds in the budget bill.