The Philippines is protesting the presence of 220 Chinese vessels near Julian Felipe Reef that is located within its exclusive economic zone, in what generals called an encroachment.
Reefs, islands and outcrops within a country's EEZ means that it has the sole right to exploit its resources. Beijing has continued building artificial islands in the West Philippine Sea, largely ignoring a UN tribunal's ruling that favored Manila and invalidated its excessive maritime claims.
“We call on the Chinese to stop this incursion and immediately recall these boats violating our maritime rights and encroaching into our sovereign territory. We are committed to uphold our sovereign rights over the WPS," Defense Secretary Delfin Lorenzana said.
Foreign Affairs Secretary Teodoro Locsin Jr said he fired off a diplomatic protest.
"We continue to monitor the situation in abidance to international laws and the preservation of the status quo in the West Philippine Sea. Our utmost priority remains to be the protection of our citizens in the area, particularly our fishermen, through increased maritime patrols," said Gen. Cirilito Sobejana, the Armed Forces chief of staff.
The Philippine coast guard detected the boats "in line formation" at the boomerang-shaped Whitsun Reef around 320 kilometers (175 nautical miles) west of Palawan Island on March 7.
The Chinese embassy in Manila did not respond to a request for comment.
A government task force charged with monitoring the contested waters announced Saturday the detection of around 220 "Chinese Maritime Militia Vessels" earlier this month.
"Despite clear weather at the time, the Chinese vessels massed at the reef showed no actual fishing activities," the agency said.
The United States has previously accused China of using maritime militia to "intimidate, coerce and threaten other nations" over its claims to almost the entire South China Sea.
The resource-rich waterway is also contested by several countries, including the Philippines.
Philippine-China relations have improved under President Rodrigo Duterte, who has tried to steer his country away from the ambit of the United States -- its former colonial master -- to pursue greater economic cooperation with its giant neighbor and American rival.
But Duterte's shift has failed to stem Chinese ambitions in the sea or unlock much of the billions of dollars of promised trade and loans.
He has repeatedly said conflict with China would be futile and that the Philippines would lose and suffer heavily in the process.
Lorenzana, however, has been more outspoken.
In August he accused China of illegally occupying Filipino maritime territory, saying the nine-dash line used by Beijing to justify its alleged historic rights to the key waterway was a fabrication.
His remarks at the time came amid a fresh row over the disputed Scarborough shoal, which China seized from the Philippines in 2012 following a tense standoff.
In 2019, the Philippines also complained after hundreds of Chinese ships were seen near Pag-asa island, also known as Thitu, which the country branded as "illegal".
-- with a report from Agence France-Presse