More than 1,000 people turned up to protest at Trump's Friday rally in San Diego, waving US and Mexican flags and carrying signs criticizing the Republican's rhetoric against illegal immigration.

After the rally was over, groups of Trump supporters began to mix with anti-Trump demonstrators, shouting and throwing water bottles at each other.

Some anti-Trump protesters scaled a barrier and threw water bottles at the police, who moved to disperse the crowd and arrested 18 people.

Unruly protesters have been common at Trump rallies in California, home to the largest Latino population in the US. San Diego, on the US border with Mexico, sees nearly 300,000 people cross legally between the two countries everyday. The city is considered bi-national by many who live and work across the border.

Trump's campaign promises, including building a wall on the border with Mexico and deporting nearly 11 million undocumented immigrants, have helped him gain supporters, but also sparked criticism.

The real estate tycoon has 1,238 Republicans on his side, one more than needed to win a Republican nomination.

mg/rc (Reuters, dpa)