Government contractors have started to build prototypes of President Trump's proposed border wall with Mexico.

Bidding documents say four of the eight prototypes are to be solid concrete and four are to be made of "other materials".

Mr Trump said on Friday that the wall should be see-through, repeating earlier claims that it should be transparent to curb the flow of drugs into the country.

"They take drugs, literally, and they throw it - 100lbs of drugs - they throw it over the wall," he said at a rally in Alabama.

"They have catapults. They throw it over the wall and it lands, and it hits somebody on the head and you don't even know they are there.


"Believe it or not, this is the kind of stuff that happens. So you need to have a great wall but it has to be see-through."

Trump: The wall is happening

A green tarpaulin hanging on a chain-link fence blocked views of the work, in a remote area of San Diego, and is expected to last 30 days.

Roy Villarreal, acting chief of the Border Patrol's San Diego sector, told reporters another contractor will evaluate each model, which will be up to 30ft (nine metres) high and 30ft long.

"It may not result in a singular winner. It may be a combination of designs being implemented," he said.

The construction site is about two miles (3km) east of San Diego's Otay Mesa border crossing at the end of steel-mesh fencing that runs from the Pacific Ocean.

The concrete designs will have openings to allow agents to see across the border.

Construction of the models is about three months behind schedule after being held up by losing bidders whose protests were eventually denied.

San Diego police and the county sheriff's department have helped set up a "free speech zone" nearby for people to demonstrate, but Mr Villarreal said he knew of no organised plans to protest.

As agents prepared for the news conference, a Mexican man breached the fence, cut his hand and was arrested.