Update 10.45pm: President Donald Trump has inspected the prototypes for the "big beautiful border wall" he wants to build to separate the US from Mexico.
The president, making his first trip to California as president, appeared engaged as he was briefed on eight border wall designs.
He said he preferred a fully concrete wall because it was the hardest to climb, but he noted that it needed to be see-through.
President Trump said the first thing he noticed on the drive to view the prototypes was the patched-up holes in part of the existing fence at the border.
"We have a lousy wall over here now, but at least it stops 90, 95%," President Trump said. "When we put up the real wall, we're going to stop 99%. Maybe more than that."
President Trump's visit was greeted with peaceful protests by demonstrators both for and against his planned wall.
The trip also came amid an escalating battle between his administration and the liberal state, which has refused to help federal agents detain immigrants in the US illegally.
The president said Tuesday that the state's sanctuary policies "put the entire nation at risk." His Justice Department sued California last week over a trio of the state's immigration laws.
"They're the best friend of the criminal," President Trump said. "That's what exactly is happening. The criminals take refuge in these sanctuary cities and it's very dangerous for our police and enforcement folks."
Demonstrations were held at the San Ysidro port of entry in San Diego, the nation's busiest border crossing, where protesters chanted, "No ban! No wall!" as honking cars and buses cheered them on.
Protests were also held on the Mexican side, in Tijuana.
President Trump was to be briefed on lessons learned from the construction of the prototypes built in San Diego last autumn.
He was also to meet with border agents and officers to ask what they need, Homeland Security spokesman Jonathan Hoffman said.
San Diego's Republican mayor criticised Mr Trump's planned short visit, saying the president will not get a full picture of the city.
Kevin Faulconer said a popular cross-border airport terminal connecting San Diego and Tijuana shows that "building bridges has worked wonders".
Mr Faulconer, writing in The San Diego Union-Tribune, also said San Diego police work to protect everyone regardless of immigration status, an apparent dig at Mr Trump's push to target illegal immigration.
Original story (7.45pm): Donald Trump arrives in California to view border wall prototypes amid protests
President Donald Trump has arrived in California to view prototypes for his "big beautiful border wall" amid protests and growing tensions between his administration and the state over immigration enforcement.
Chanting "No ban. No wall", demonstrators were cheered on by honking cars and buses at the San Ysidro port of entry in San Diego, the nation's busiest border crossing.
President Trump's visit coincided with an escalating war of words between his administration and the liberal state, which Democrat Hillary Clinton easily won in the 2016 presidential election.
California officials have defiantly refused to help federal agents detain and deport immigrants in the US illegally, and the Justice Department sued the state last week over three of its immigration laws.
The president, who arrived in San Diego on Tuesday afternoon, planned to inspect eight towering prototypes for the wall in an area of the border heavily cordoned off and far from the rallies on the US side.
Protests were planned on the Mexican side, too, in Tijuana. Trucks were parked in between the row of prototypes and the border, blocking the view from Mexico.
Demonstrators said they planned to later line up and greet people walking into the United States at the San Ysidro crossing to show Americans welcome immigrants.
President Trump is expected to be briefed on lessons learned from the construction of the prototypes built in San Diego last autumn.
He will meet with border agents and officers to ask what they need, Homeland Security spokesman Jonathan Hoffman said.