Duterte enjoys firm popular backing but his setting aside of the standoff with China over the resource-rich waterway is criticized as weakness by some in the Philippines.

The issue has flared up since a Chinese fishing trawler hit and sank a Filipino boat on June 9 near Reed Bank, an area that is within Manila's territory but which is also claimed by Beijing.

After a string of small street protests, as well as criticism from opposition politicians and former officials, Duterte lashed out while talking to reporters late Thursday.

"Impeach me? I will arrest all of them. I dare you to do it," Duterte said.

"I tell these stupid people, I said I deal with reality," he added.

Critics had raised the spectre of impeachment after Duterte, responding to the sinking controversy, said he allowed Chinese fishermen in Philippine waters because "we're friends".

Opposition politicians allege that violates a provision of the Philippine constitution mandating the government to protect its marine wealth, including its exclusive economic zone, "and reserve its use and enjoyment exclusively to Filipino citizens."

Duterte enjoys sky-high public popularity in the Philippines despite international disapproval of key policy measures, like his narcotics crackdown that has killed thousands.

His allies will hold the majority in the upcoming session of the legislature, which would be the authority in impeachment proceedings against any Philippine official.

However, segments of the public and press question whether Duterte's pivot toward China has brought too little in hoped-for investment and infringed too much on the nation's sovereignty.

China claims most of the waterway, through which billions in trade passes annually, and has rejected a 2016 international tribunal ruling that its claim was without basis in law.

A 1982 United Nations treaty on the law of the sea gives coastal states like the Philippines jurisdiction in exploring and exploiting marine resources over their exclusive economic zone, including waters extending 322 kilometres (200 miles) from the shore.

Twenty-two Filipino fishermen were rescued by Vietnamese fishermen hours after the June 9 collision, in which Manila initially alleged the Chinese vessel left Filipinos dead after their boat sank.

Duterte has tried to downplay the case, calling it an "accident" and accepting Beijing's offer to conduct a joint investigation.

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