About 50 protesters, most of them students, reached Thitu island in the Spratly archipelago on Saturday in a stand against what they say is Beijing's creeping invasion of the Philippine exclusive economic zone, said Eugenio Bito-onon, the island's mayor.
China claims almost all the South China Sea, believed to have huge deposits of oil and gas, through which about $5 trillion in ship-borne trade passes every year. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.
China was "strongly dissatisfied" by what the Filipinos had done, Chinese Foreign Ministry spokesman Lu Kang said, reiterating that China has indisputable sovereignty over the Spratly islands.
"We once again urge the Philippines to withdraw all its personnel and facilities from the islands that it is illegally occupying, refrain from actions that are detrimental to regional peace and stability and not conducive to Sino-Philippines relations," Lu said.
Describing their expedition as a "a patriotic voyage", the protesters, led by an ex-marine captain, plan to camp on the island, which the Philippines calls Pagasa, for three days in a symbolic act of defiance.
Government and military officials had tried to prevent the group from sailing to the disputed waters, citing security and safety reasons after a storm in the South China Sea earlier this month.
The Philippines was also concerned about China's reaction to trip as Manila has been trying to calm tensions heightened by Beijing's rapid expansion in the South China Sea - building seven artificial islands in the disputed waters.
The Philippines has challenged Beijing before the arbitration court in The Hague, a case Beijing has not recognized.Last modified on Monday, 28 December 2015