SAN DIEGO –

Donald Trump brought his message of walls and deportations to San Diego, the doorstep of America's busiest border crossing, on Friday amid one of the largest protests against the presumptive Republican presidential nominee.

Shortly before taking the stage, Trump also issued a statement ruling out a one-on-one debate with second-place Democratic hopeful Bernie Sanders.

The scene inside the San Diego Convention Center during Trump's speech was relatively placid. Outside, demonstrators opposed to his controversy-ridden White House bid marched and chanted, carrying signs criticizing his rhetoric against illegal immigration.

Waving U.S. and Mexican flags, more than 1,000 people turned out for anti-trump rallies in San Diego, a city on the U.S.-Mexico border whose San Ysidro port of entry sees nearly 300,000 people a day cross legally between the countries.

San Diego is considered a binational city by many who live and work on opposite sides of the border, and about a third of the city's population is Latino.

During Trump's speech, some protesters outside the convention center scaled a barrier and lobbed water bottles at police. One man was pulled off the wall and arrested as others were surrounded by fellow protesters and backed away from the confrontation.

After the convention center emptied, clusters of Trump supporters and protesters began to mix in the streets, many exchanging shouted epithets and some throwing water bottles at one another.

Police in riot gear declared the gathering an unlawful assembly and ordered the crowd to disperse, herding the crowd out of the Gaslamp Quarter of hotels and restaurants.

Police reported making more than dozen arrests during the hour following the department's order to disperse.

Trump has weathered months of blowback from all ends of the political spectrum for his immigration policy, which calls for the building of a wall along the U.S.-Mexico border and deporting the nearly 11 million undocumented immigrants who reside in the United States.

Critics have said his plan is needlessly cruel and impossible to implement. At Trump's campaign stops, attendees often chant, "Build the wall."

While Trump is running unopposed in the June 7 California Republican primary, his stance on border control and deportation seems unlikely to resonate with the electorate at large in a state where political fallout from a Republican-backed crackdown on illegal immigrants 20 years ago cost the party dearly.

Friday was not the first time Trump has been greeted by civil unrest in California, which is home to the largest Latino population in the country. Late last month, a visit to the California Republican convention set off days of protests in the area, leading to several arrests.

Trump's rejection of a debate with Sanders, who was also in California on Friday, killed off a potentially high-ratings television spectacle.

The suggested debate, an idea first raised during a talk show appearance by the New York billionaire, would have sidelined likely Democratic nominee Hillary Clinton but given Sanders a huge platform ahead of California's Democratic primary.

A day after saying he would welcome a Sanders debate, Trump called the idea "inappropriate," declaring that he should only face the Democrats' final choice.

"I will wait to debate the first-place finisher in the Democratic Party, probably Crooked Hillary Clinton," Trump said in a statement.

Sanders, a U.S. senator from Vermont, expressed disappointment on Friday, and sought to goad Trump into reconsidering.

"Well, Mr. Trump, what are you afraid of?" he said in a video clip posted on ABC News' Twitter account.

Trump suggested broadcast networks were unwilling to go along with his demand that at least $10 million raised from the encounter be donated to charity. "I'd love to debate Bernie," he told a rally in Fresno, California. "But the networks want to keep the money for themselves."

Sanders is trailing Clinton in the race to secure their party's nomination, but opinion polls show he is slicing into her lead in California.

Clinton has shown no interest in debating Sanders before the California primary, which will be part of a final slate of nominating contests. It is possible she will clinch the nomination by winning New Jersey earlier that day, making the outcome in California superfluous.

The former U.S. secretary of state has said she is looking forward to debating Trump later this year ahead of the Nov. 8 general election.

Clinton leads Trump by 4 percentage points in the most recent Reuters/Ipsos poll. Democrats nationally remain evenly split between Clinton and Sanders.