Trump warns of 'bedlam' without border wall with Mexico

US President Donald Trump warned on Tuesday there would be "bedlam" without the controversial wall he wants to build on the border with Mexico, as he inspected several prototype barriers.

The trip to the "Golden State" – the most populous in the country and a Democratic stronghold – was largely upstaged by his own announcement that he had sacked Secretary of State Rex Tillerson.

On the other side of the border, a small group of anti-Trump protesters vented their frustration, and announced plans to boycott US businesses over the frontier.

"For the people who say 'no wall' – if you didn't have walls over here, you wouldn't even have a country," Trump said near the border in San Diego.

Trump repeated his insistence that law enforcement personnel should be able to see through the structure so that they could monitor criminal cartels that might be "two foot away" on the Mexican side.

"Without a wall there would be bedlam, I imagine," he added.

Trump inspected eight 30-foot high full-scale models made of concrete and steel, erected side-by-side at Otay Mesa, an area in southern San Diego along the border with Tijuana, Mexico.

Each prototype cost more than $300,000 and, according to some estimates, the complete wall could carry a $20 billion price tag.

Environmental catastrophe

Congress has yet to approve the funding amid skepticism and Democratic opposition, but an administration official said the wall would save far more money than it cost.

California has been at the forefront of resistance to the Republican leader's anti-immigration agenda and at odds with his stance on a number of other issues, from gun control to marijuana and the environment.

Protests on Mexican side of the border

Before Trump's visit, demonstrators protested against his migration policies on the Mexican side of the border in Tijuana. 

On the Mexican side, federal police had to persuade around 50 demonstrators not to burn a yellow-haired pinata in Trump's likeness.

The hulking prototypes can be seen from across the border in Tijuana, where residents are not overly impressed with the real estate tycoon, who launched his presidential campaign calling Mexicans "criminals" and "rapists."

Ahead of the visit, a few dozen people gathered for a pro-Trump rally on the US side of the border near the prototypes, while a similar number of the president's detractors came to the San Ysidro border post.

Demonstrators draped themselves in US flags and waved placards emblazoned with slogans such as "Humpty Trumpty will fall off his wall," "Resist unstable idiot," and "No hate in the Golden State."

The border with Mexico stretches nearly 3,200 kilometres and about a third of it already has some type of barrier or wall.

Trump's insistence that Mexico pay for the wall has soured relations and Mexican President Enrique Pena Nieto recently canceled plans to visit Washington amid continuing disagreement.

Trump is expected to wrap up his visit with an evening of fundraising in Beverly Hills to raise money for his 2020 re-election campaign.