Carrying placards declaring "Dump Trump" and "Down with U.S. Imperialism", the left-wing protesters were blocked by police in riot gear with shields and batons, and then showered with jets of water from a fire engine.
The P15.5-billion budget for the Association of Southeast Asian Nations (ASEAN) Leaders' Summit should have been given to starving and dying Filipinos across the country, the progressive solon said.
US President Donald Trump arrived in the Philippines on Sunday for meetings with his Asia Pacific counterparts, the last stop of his Asian tour. "One fight there, another here", he said this week, in Vietnam ahead of the recent Asia-Pacific Economic Cooperation (APEC) meeting. "I'm a very good mediator and arbitrator".
"If I can help mediate or arbitrate, please let me know", Mr Trump said.
The ASEAN Summit this year under the chairmanship of the Philippines would make a forward movement to the start of substantive negotiations with China for a Code of Conduct (CoC) that would prevent "escalation" of the maritime dispute in the South China Sea but there need not be any "non-adversarial" resolutions against Bejiing though.
The controversial issue was also understood to have figured in a meeting of officials from India, the US, Japan, and Australia under the proposed Quadrilateral coalition of the four countries.
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On Monday, the 31st ASEAN Summit and Related Summits will be officiated by Duterte at the Cultural Centre of the Philippines before the leaders gather at the Philippine International Convention Centre for a series of meetings, including the 40th anniversary of ASEAN-US Dialogue Relations.
China insists that the territorial issues should be handled directly between countries in the region.
Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have conflicting claims there.
The Philippines has historically been the United States' strongest ally in the region. The South China Sea will likely be a hot topic throughout the summit, as most claimants will be attending.
Tensions in the contested waters have ratcheted up since 2014 as China has turned sandbars into islands, equipping them with airfields, ports and weapons systems and warned United States warships and aircraft to stay away from them.
India has been supporting freedom of navigation and access to resources in the South China Sea in accordance with principles of global law, including the 1982 UN Convention on the Law of the Sea.