Former Philippine foreign minister Albert del Rosario was denied entry to Hong Kong on Friday, in what critics called retaliation for his attacks on Beijing's contested claims over the South China Sea.
Del Rosario was behind two prominent legal initiatives against China, including a 2013 case at an international tribunal which ultimately ruled against Beijing's claim to most of the resource-rich waterway.
His deportation comes as anger is still bubbling in Hong Kong over a proposed bill that would have allowed extraditions to the mainland, raising fears of people being ensnared in China's opaque court system.
Del Rosario said he flew to Hong Kong early Friday using a Philippine diplomatic passport but was taken to an immigration holding area on arrival, where he remained "for nearly three and a half hours".
Del Rosario's lawyer Anne Marie Corominas subsequently told AFP: "He's been excluded and deported." She said that authorities had given no reason for denying him entry, and he was put on a flight back to the Philippines. He landed in Manila Friday evening.
China has yet to offer an explanation, but Beijing's foreign affairs ministry spokesman Lu Kang told reporters: "Who is allowed or not allowed to enter the country is entirely China's sovereignty".
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Although Hong Kong was returned from British to Chinese rule in 1997, it is still administered separately under an arrangement known as "One country, two systems".
But activists have been alarmed in recent years by what they feel is a tighter grip by Beijing, though Hong Kong still retains freedoms unseen on the mainland.
In March, del Rosario filed a complaint against Chinese leader Xi Jinping at the International Criminal Court, alleging "crimes against humanity" over the supposed environmental fallout of Beijing's activities in the disputed waters.
Philippine President Rodrigo Duterte has largely set aside the nation's once tense standoff with China over the South China Sea, opting instead to court Beijing's investment and trade.
The Philippines has claims over parts of the key waterway, where China has staked "indisputable sovereignty" and built artificial islands with military facilities and airstrips.
Duterte's strategy has prompted sharp criticism from the opposition and segments of public, charging the president with frittering away the nation's sovereignty.
Hong Kong immigration authorities briefly held another Philippine critic of China, former Supreme Court justice and special anti-graft prosecutor Conchita Carpio Morales on May 21 when she made a private visit to the territory with her family.
Morales, who was also a party in the complaint filed by del Rosario at the ICC, was approved for entry hours later, but chose to return to Manila instead.
"Hong Kong's questioning of secretary Albert del Rosario... is a detestable retaliatory act of China as it confirms that Hong Kong has shed or been cowed into giving up its autonomy," she told AFP.
Del Rosario, who served under Philippine president Benigno Aquino from 2011-16, accused airport immigration authorities of acting "in violation of the Vienna Convention" on diplomatic privileges.
Friday's incident comes as the Duterte government seeks to calm public outrage over the June 9 sinking of a Filipino fishing boat by a Chinese vessel in an area of the South China Sea claimed by both countries.
Duterte has faced criticism for not taking China to task over the incident, which is a test of his Beijing-friendly policy.