Hundreds of protesters clashed with anti-riot police on Tuesday while attempting to march toward the U.S. embassy in Manila.
The protesters gathered just a few hours before the Asia-Pacific Economic Cooperation (APEC) Leaders' Meeting began late Tuesday.
They attempted to march to a major road leading to the venues of the summit, but were blocked by police who erected impromptu fences.
Riot police scuffled with the demonstrators, who were chanting activist slogans and waving anti-APEC banners. Some of the protesters fainted in the ensuing melee.
The demonstrators criticized what they said was the APEC's imperialist agenda.
A spokesman for the protesters accused the Filipino government of being a "puppet" of the United States and President Barack Obama.
Human rights and pro-labor groups have been protesting against the APEC summit, saying it is a vehicle for corporations and rich nations to protect their interests at the expense of smaller economies.
APEC, which accounts for 60 percent of global output and nearly half of world trade, is aiming for a larger free-trade area for its 21 economies by 2025. A re-emergence of protectionism in some states as growth slows, however, could be a hindrance.