A group of Filipino protesters has landed on a disputed Philippine-held island in the South China Sea, a local government official said on Sunday, in a risky expedition that may trigger a strong reaction from China.

About 50 protesters, most of them students, reached Pagasa 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.

"The 'freedom voyage' arrived at about 8:30 a.m. on Saturday from Balabac island on a motor launch," Mr. Bito-onon told Reuters, adding the protesters left southern Palawan on Thursday in fine weather to make the long sea crossing.

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 on the strategic waters.

Describing their expedition as a "a patriotic voyage," the protesters, led by an ex-marine captain, planned to camp on Pagasa for three days in a symbolic act of defiance against China.

"We encourage the highest leadership of the country to inform the people correctly without sugar coating the truth about Chinese invasion of our exclusive economic zone," the protesters said in a post on Facebook.

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 the 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.

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The Philippines has challenged Beijing before the arbitration court in The Hague, a case Beijing has not recognized.

A spokesman for Philippine President Benigno Aquino said in a radio interview on Sunday the military was closely monitoring the trip and would assist the protesters if necessary.