Protests turned violent in California when groups wearing black clothing and masks attacked right-wing protesters.

Left and right-wing groups were protesting in Berkeley on Sunday after a planned anti-Marxism demonstration was cancelled due to safety concerns.

As protesters chanted "No Trump, No KKK, No fascist USA," conservative rally organisers showed their support for President Donald Trump.

The protests were largely peaceful until members of the antifa, or anti-fascists, overran police barricades around the protest area at Civic Center Park.

In the brief scuffles that followed six people were injured and 13 were arrested on various charges including assault with a deadly weapon.

Among those targeted was Joey Gibson, leader of the right-wing organisation Patriot Prayer that had called off a demonstration a day earlier in San Francisco. Anarchists pepper-sprayed him and chased him as he backed away with his hands held in the air.

Separately, groups of hooded, black-clad protesters attacked at least four other men, kicking and punching them until police stepped in.

Groups that planned the counter-demonstrations were concerned that white nationalists might show up and there would violence like the kind two weeks ago in Charlottesville, Virginia, where a woman was killed.

No white nationalists showed up and only a handful of pro-Trump demonstrators were visible in the crowd.

Police pulled one supporter of Trump out of the park over a wall by his shirt as a crowd of about two dozen counter-demonstrators surrounded him and chanted "Nazi go home" and pushed him towards the edge of the park.

At least two people were detained by officers for wearing bandannas covering their faces.