Trump signed two executive orders on Wednesday that follow his campaign promises to be tougher on illegal immigration and build a wall along the border.
He is also moving to cut federal grants for "sanctuary cities" that protect undocumented immigrants and he is adding funding for immigration enforcement, including 10,000 new immigration enforcement officers.
"A nation without borders is not a nation," Trump said. "Beginning today the United States of America gets back control of its borders, gets back its borders."
During the 2016 presidential campaign, Trump promised that Mexico would pay to build the wall. Now his plan is to have U.S. taxpayers fund the wall, but to seek reimbursement from Mexico.
People along both sides of the U.S.-Mexico border at San Ysidro expressed skepticism about the plan.
"They can build a wall but the wall's not gonna keep 'em out," said Robert Trent, an American retiree living in Mexico. "It's a waste of money if you ask me, because they'll just go under."
California Democratic lawmakers also vowed to fight the president's plans. They are pushing legislation that would provide funding for legal representation for undocumented immigrants and that would prevent state and local law enforcement from acting as immigration agents.
Los Angeles County Sheriff Jim McDonnell said Trump's orders will not change his department's policy preventing deputies from questioning people about their immigration status or operating as immigration-enforcement officers.
"The executive order does not change the mission of the Los Angeles County Sheriff's Department," McDonnell said. "Our priority continues to be protecting our public. We accomplish this by gaining the trust of our communities and following the California Trust Act of 2014 and the Truth Act of 2016. Our department policy clearly states that our deputies do not ask for one's immigration status. Immigration enforcement remains a federal responsibility."
Los Angeles Mayor Eric Garcetti said the city plans to fight the executive orders in court.
"We are here to say we will protect our people," Garcetti said.