US police and anti-Trump demonstrators have clashed at a Donald Trump rally in southern California.

San Diego police declared the gathering of about 1,000 anti-Trump protesters illegal when the crowd's behaviour became "unlawful".

Police moved to disperse the crowd after bottles were thrown, and 18 people were left seeking medical attention before calm was restored.

Trump's outspoken comments, including the presidential campaign promise to build a wall on the border with Mexico, have not only garnered popular support but also sparked virulent criticism.