Police officers in riot gear declared a protest against Donald Trump unlawful as they made around dozen arrests outside a rally in California.
The presumptive Republican nominee was speaking to an enthusiastic crowd of supporters in San Diego's convention centre.
But outside around 1,000 people demonstrated against the billionaire tycoon, in tense scenes which included police pepper spraying some of the protesters.
Police arrested one man who climbed a railing separating protesters from officers. His arrest led demonstrators to spray water and throw empty plastic bottles at officers.
In another tense exchange, police shoved back demonstrators to separate them from Trump supporters when they left the centre after the rally.
Police declared the gathering unlawful after shouting matches erupted and people began hurling rubbish at each other.
At an earlier rally in Fresno, Trump told California voters that he can solve their water crisis, declaring: "There is no drought."
California is the biggest agriculture producer in the US and is in the middle of a drought, raising the stakes in water disputes among farmers, cities and towns, and environmental interests.
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".
He told thousands of supporters at the rally: "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."