Street clashes broke out on Wednesday shortly before APEC forum foreign and trade ministers opened two days of talks.
In the Chilean capital, helmeted military-style police with plastic shields fired water cannon as the demonstrators gathered along the central Alameda boulevard, hoping to march to the central library.
There appeared to be several hundred demonstrators in dispersed groups.
No injuries were immediately reported.
Students, some from secondary school, cut off a side street, building barricades with trash cans, and threw paint bombs at police vehicles, according to an AFP photographer.
Police hauled away dozens of the anti-APEC, anti-Bush protesters.
Powell is expected to use the
conference to focus on Pyongyang
"They won't let us protest because they want to show a false image (to the APEC leaders)," said one demonstrator, student Sebastian Valenzuela.
The conference centre, far from the centre of Santiago, was protected by mounted military style police. Security forces, with side-arms but no automatic weapons visible, checked entrants and their bags.
Policymakers were discussing counter-terrorism measures, how to spur flagging efforts to tear down global trade barriers, and whether to consider an Asia-Pacific free trade agreement, according to a copy of their agenda obtained by AFP.
North Korea was not on the official agenda.
But US Secretary of State Colin Powell, who resigned on Monday and will be succeeded by national security advisor Condoleezza Rice, planned to apply regional pressure to Pyongyang, which has refused to join six-country talks to end its nuclear weapons drive.
"They won't let us protest because they want to show a false image (to the APEC leaders)"
"(We will) make sure that we use our alliances in Asia and the partnerships we have in Asia to keep pressing to find a solution to the North Korean nuclear programme," he told a news conference this week in Washington.
Bush is pushing his core agenda, seeking support for the "war on terror" and the Iraq conflict.
On the economic debate, Asia-Pacific leaders appeared split on a radical proposal by business chiefs for APEC leaders to consider the creation of a region-wide free trade agreement.
APEC is already working on a decade-old set of non-binding promises, made in Bogor, Indonesia, to demolish barriers to trade and investment by 2010 in its developed economies and by 2020 in developing economies.
APEC comprises Australia, Brunei, Canada, Chile, China, Hong Kong, Indonesia, Japan, Malaysia, Mexico, New Zealand, Papua New Guinea, Peru, the Philippines, Russia, Singapore, South Korea, Taiwan, Thailand, the United States and Vietnam.