Duterte's unpredictable foreign policy is also expected to be in focus at the Association of Southeast Asian Nations (ASEAN) event, with confusion surrounding his approach to Chinese expansionism in the South China Sea.
It will be closely watched for how Duterte, who has shocked with curse-laden tirades against the United States and the United Nations, handles hosting his first major diplomatic event.
"This will be a very interesting time. Everyone thinks he is a very charismatic leader, he speaks off the cuff," a Southeast Asian diplomat involved in the summit's preparations told AFP.
Duterte has relentlessly railed against the mostly Western critics of his drug war, angrily demanding they respect him while in turn using abusive terms to describe them.
Duterte hogged the headlines at another ASEAN leaders' summit in Laos last year when he called then US president Barack Obama, who was also attending, a "son of a whore" for criticising the drug war.
The mood will likely be far calmer in Manila for the two-day annual event starting Friday, with Duterte to enjoy drug war backing from most of his guests, according to observers.
"The region is falling deeper into dictatorship, repression and rights abuse."
Among the heads of undemocratic regimes who will convene in Manila are Thai military junta chief Prayut Chan-O-Cha and Sultan Hassanal Bolkiah of Brunei, as well as the leaders of Laos and Vietnam.
Cambodia's Hun Sen, a former Khmer Rouge cadre, and Malaysian Prime Minister Najib Razak, who has faced allegations of corruption and vote rigging, will also attend.
"I expect them to support the efforts of the Philippines to curb the drug menace, period. Without talking about how we achieve that," Baja told AFP.
Baja said that, even if leaders did disapprove of Duterte's tactics, they would observe a long-standing ASEAN policy of non-interference in each others' affairs.
Meanwhile, Duterte's contradictory statements on the Philippines' approach to its dispute with China over rival claims in the South China Sea will be under close scrutiny.
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