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

Construction of eight models in a remote area of San Diego is about three months behind schedule after being held up by losing bidders whose protests were eventually denied.

A green tarpaulin hanging on a chain-link fence blocked views of the work, which is expected to last 30 days.

Roy Villarreal, acting chief of the Border Patrol's San Diego sector, said another contractor will evaluate each model, which will be up to 9 metres high and 30 feet long.

"It may not result in a singular winner. It may be a combination of designs being implemented," Mr Villarreal told reporters next to existing border fencing.

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

The construction site is about 3 kilometres east of San Diego's Otay Mesa border crossing at the end of steel-mesh fence that runs from the Pacific Ocean.

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.

Bidding documents say four of the 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, apparently at odds with the initial instructions for solid concrete.

Mr Villarreal said the concrete designs would be built with openings to allow agents to see across the border.

The administration faces several federal lawsuits in San Diego that seek to block the prototypes and plans to replace existing barriers in California.

A complaint filed last week by California Attorney General Xavier Becerra, a Democrat, largely mirrors two others by environmental advocacy groups that allege the administration overstepped its authority to speed up construction of the wall.

At issue is a 2005 law that gave the Homeland Security secretary broad powers to waive dozens of laws for border barriers, including the National Environmental Policy Act, Clean Air Act and Endangered Species Act. The lawsuits say that authority has expired.