Some of Australia's worst racial violence in history erupted on Sunday as thousands gathered on Cronulla Beach, Sydney.

The crowd targeted people of Arabic and Mediterranean appearance, blamed for attacks on two lifeguards a week ago.

Police and ambulance workers were pelted with beer cans and bottles. Trouble spread overnight to other suburbs.

Police rushed to protect people at risk of assault, as the alcohol-fuelled violence left at least 30 people injured.

More than a dozen people were arrested as police sought to contain the rioters, many of whom chanted racist slogans.

Authorities and Muslim leaders have condemned the riots, said by police to have been fanned by white supremacist groups.

Prime Minister John Howard said the "totally unacceptable" violence did not reflect an underlying racism in Australia.