The activists carried placards saying "The Philippines is ours, get out China," "Hands off Philippines" and "China out of the Philippine Waters" as they marched to the embassy in the financial district of Makati City.
"We are confident the arbitration court will favour the Philippines and we can disprove China's irrational claim in our territory," said Salvador France, vice chairman of Pamalakaya, a fisherfolk organisation that led the protest.
"China by hook or by crook must immediately stop their reclamation activities in the West Philippine Sea and leave if the arbitration will favour the Philippines' claim on the disputed waters," he added.
The Permanent Court of Arbitration is scheduled to hand down a decision on the Philippines' case against China at 5 pm (0900 GMT).
Manila filed the case in January 2013 after China took control of Scarborough Shoal, 124 nautical miles from the Philippines' northwestern coast.
The case seeks to declare China's claims to almost the entire South China Sea as illegal and against international maritime laws, particularly the UN Convention on the Law of the Sea.
Beijing has refused to participate in the arbitration.
Apart from China and the Philippines, Brunei, Malaysia, Taiwan, and Vietnam also have overlapping claims to the South China Sea, a key shipping lane believed to be rich in mineral and marine resources.
- DPA