SAN DIEGO, Calif. -- Presumptive Republican presidential nominee Donald Trump told California voters Friday that he can solve their water crisis, declaring, "there is no drought."

Speaking at a rally in Fresno, Calif., Trump accused state officials of denying water to Central Valley farmers so they can send it out to sea "to protect a certain kind of three-inch fish."

"We're going to solve your water problem. You have a water problem that is so insane. It is so ridiculous where they're taking the water and shoving it out to sea," Trump said to cheers at a rally that drew thousands.

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Trump's rallies Friday in Fresno and San Diego drew vocal crowds of protesters under heavy police presence.

More than a thousand Trump opponents demonstrated outside San Diego's convention center, some engaging in shouting matches with the candidate's supporters.

Many protesters carried signs critical of his plan to wall off the U.S. border with Mexico, which runs just south of the city. Some waved Mexican flags and one man broke apart a Trump piñata with a pole.

Anti-Trump protesters protest outside a venue where republican presidential candidate Donald Trump will speak in San Diego, California on May 27, 2016. MARK RALSTON/AFP/Getty Images

Numerous opposition groups, from organized labor to immigration advocates, marched to the center and demonstrated outside, CBS affiliate KFMB reported. Protests turned violent Tuesday in Albuquerque, leading to concerns about security in San Diego.

Police Chief Shelley Zimmerman told reporters earlier Friday that law enforcement would not tolerate violence, disruptive behavior or unlawful acts at the Trump event.

"The safety of our public is paramount," Zimmerman said. "The whole goal is to provide a safe environment for everyone."

The department had a "significant presence" of uniformed and plainclothes officers at the bayside gathering and has coordinated with state, local and federal agencies to promptly put a stop to any combative or destructive protesters or counter-protesters, the chief said.

The city designated demonstration zones -- one for Trump supporters and another for those who oppose the 69-year-old billionaire's presidential bid -- on the northwest and southeast sides of the Harbor Drive center and will strictly monitor attendees' behavior while ensuring their constitutional free-speech rights, Zimmerman said.

San Diego police said a total of 35 people were arrested, according to KFMB. Officers were seen beating a man with batons after he attempted to scale a security barricade, CBS News correspondent Carter Evans reported.

Police arrest a person outside a Donald Trump rally in San Diego, California on Friday, May 27, 2016. CBS News

Some people in the crowd eventually started throwing things, prompting police to declare an unlawful assembly and begin moving people out of the area. Police in riot gear maintained a huge presence as the crowd moved into the city's Gaslamp Quarter, KFMB reported.

#SDPD disperses crowds after Trump rally: fights broke out, arrests made, pepper spray deployed: @CBS8 at 6:30 pic.twitter.com/btz8X1yFJv — Richard Allyn (@RichardCBS8) May 28, 2016

In the protest area, Martha McPhail of La Mesa held a "Dump Trump" sign.

"I am opposed to the hateful, bigoted, racist language of Donald Trump and his arrogance and intolerance," McPhail told City News Service. "I'm for all of our people -- all races, sexes, genders, military veterans -- and he's divisive."

Bryan Sanders, a Tucson resident, said he came to San Diego because Trump needs to be stopped.

"Donald Trump is a fascist," said Sanders, 33. "He has no business being in American politics. He's un-American. His march toward power needs to stop, and it's gonna."

Sanders said he was injured at a previous Trump rally in his hometown.

Riley Hansen, a 19-year-old supporter, was selling pro-Trump T-shirts and said he likes the candidate's business knowledge.

"My dad always told me you need a businessman as president," the Orange County resident said. "I like his policies. We need the border wall built."

Tami Eshelman of Lake San Marcos said she came to hear Trump speak and to show her support. She said she wants to find out who would serve on his team, such as cabinet members, if he's elected.

Trump has reached the required number of delegates to officially clinch the Republican nomination, according to a CBS News estimate.

Earlier Friday in Fresno, Trump said he spent 30 minutes before the rally meeting with more than 50 farmers who complained to him about their struggles.

"They don't understand -- nobody understands it," he said, adding that, "There is no drought."

Protesters yell as police officers form a skirmish line outside a rally for Republican presidential candidate Donald Trump in Fresno, California, May 27, 2016. REUTERS/Noah Berger

Trump appeared to be referring to water that runs naturally from the Sacramento River to the San Francisco Bay and then to the ocean. Some farmers want more of that flow captured and diverted to them. The three-inch Delta smelt is a native California fish on the brink of extinction. The smelt has become an emblem in the state's battles over environmental laws and water distribution.

Politically influential rural water districts and well-off corporate farmers in and around California's Central Valley have been pushing back against longstanding federal laws protecting endangered fish and other species, saying federal efforts to make sure endangered native fish have enough water is short-changing farmers of the water they want and need for crops.

Water authorities say they can't do it because of the water rights of those upstream of the farmers, and because of the minimum-water allowances needed by endangered species in the bay and by wildlife in general.

The farm lobby, a heavyweight player in California's water wars, also is seeking federal and state approval for billions of dollars in new water tunnels, dams, water distribution plans, and other projects.

Trump said, "If I win, believe me, we're going to start opening up the water so that you can have your farmers survive."

California is the country's No. 1 agriculture producer. The state's five-year drought is raising the stakes in water disputes among farmers, cities and towns, and environmental interests. California last year marked the driest four-year period in its history, with record low rainfall and snow.