Listing some of Manila's actions against United Nations envoys, including reportedly filing terrorism charges against one, rights chief Zeid Ra'ad Al Hussein said "it makes one believe that the president of the Philippines needs to submit himself to some sort of psychiatric evaluation".
A series of profanity-laced diatribes lashed out by President Rodrigo Duterte of the Philippines against United Nations rapporteurs has forced a scathing reply by UN human rights chief who says the leader needs "psychiatric evaluation".
Tauli-Corpuz denounced the government, calling the complaint "baseless, malicious and irresponsible".
"Human rights, human rights, son of a bitch, human rights", Duterte said angrily in a mix of Filipino and English.
Referring specifically to Duterte's comments about Callamard, Zeid said: "This is absolutely disgraceful that the president of a country could speak in this way, using the foulest of language against a rapporteur that is highly respected".
Al Hussein also hit out at President Duterte, saying: "He needs to submit himself to some sort of psychiatric examination".
"Government accusations that worldwide human rights experts have links to the NPA seek to undermine the valuable work they do to promote accountability for rights abuses", Asia division researcher for HRW Carlos Conde said in a statement.
But the peace process was called off in November after what Duterte called repeated attacks by the NPA during talks.
Conde said HRW saw the 55-page petition the government filed in a Manila court in February.
But the petition also sought to include a UN Special Rapporteur, a former lawmaker, four former priests and at least 600 individuals as terrorists.
Worldwide rights groups and local critics have accused Duterte of drifting toward authoritarianism after declaring martial law in the south during a major attack by pro-ISIS (Islamic State of Iraq and Syria) group militants previous year.
His concerns were echoed by Michel Forst, the UN's special rapporteur on the situation of human rights defenders.
"There's a long history in the Philippines of the state security forces and pro-government militias assassinating people labeled as NPA members or supporters", he added.
"I deplore President Duterte's statement last week to elite police units that they should not cooperate "when it comes to human rights, or whoever rapporteur it is" and the continued vilification of this Council's Special Rapporteur on extra-judicial killings by the authorities", Zeid said.
Also on the list was former congressman Satur Ocampo, who faces a criminal case over the murder of suspected military spies in the communist movements in the 1980s.
There was no basis for the charge of terrorism, said Mr Sison, who was a mentor of Mr Duterte when he was at university, although the two are now bitter rivals.
In a statement, Sison said Duterte was launching an anti-communist witch-hunt "to silence his critics and opposition to his cruel and corrupt regime, and in order to realise completely his fascist dictatorship". "It is Duterte who is truly the No 1 terrorist in the Philippines", he said.
Since then, Duterte has been vicious in his attacks on the communists and his recent controversial comments to "shoot women in the vagina" were directed at female communist rebel fighters.