About 3,000 people took to the streets in Sydney despite cold and rainy weather in the vicinity of Sydney's Hyde Park, where the forum of 21 Pacific Rim countries has reached its summit stage.
The police said those arrested would face charges for contempt of law enforcement officers, throwing various things at them and for resistance. Two policemen were injured and two protestors were detained for public nudity.
The protestors largely represent anti-war organizations and are mainly targeting U.S. President George W. Bush's foreign policy, demanding an end to the Iraq war, U.S. disarmament and a halt of uranium enrichment.
The police have cordoned off the streets leading to the Sydney Opera House, where the APEC leaders are meeting, and are pushing the protestors back.
Earlier, they resorted to using a water cannon to disperse a group wearing black, a hallmark in past demonstrations of violent anarchist groups, who gathered near Town Hall.
Representatives of the Vietnamese and Tibetan diasporas are also rallying in another park, demanding that the forum focus on human rights in China.
Sydney has seen an unprecedented number of policemen - about 5,000 - and helicopter patrols in the last few days, reflecting a fear of riots similar to those that erupted in Melbourne last year when it hosted a forum of the Group of Twenty.