Giacomo Filibeck, a deputy secretary-general of the PES, was denied entry at the Nactan-Cebu International Airport in the central Philippines for "illegal political activity."
Filibeck was to attend a two-day party congress of the local Akbayan Party, a sister party of the PES. He arrived at the airport with about 20 other foreign delegates but was stopped at the immigration counter.
Filibeck, an Italian Socialist Party official who was part of the former ruling party in Rome, was given a slip of paper informing him that his name was on a blacklist order, according to a statemement put out by the Akbayan Party.
The other foreigners were allowed entry.
Filibeck was in the Philippines in October 2017 as part of an international human rights fact-finding mission which condemned Duterte's deadly anti-drug campaign.
Government 'paranoid'"Akbayan condemns, in the strongest terms, the detention and deportation of European human rights leader Giacomo Filibeck," Akbayan said in a statement.
"This unfortunate incident only shows how paranoid this government is in keeping the rest of the world blind from the damage President Duterte has done to our country," it said.
Back in Europe, PES also protested "in the strongest possible terms the unjustified detention and deportation" of one of its senior officials
Read more: Duterte threatens to eject EU ambassadors
Under international scrutinyPES President Sergei Stanishev said it was "unacceptable" that a member of the party "should (be) treated as a criminal on orders of the government and forcibly deported from the country."
"Clearly, the president intends to silence criticism of his deadly policies both at home and abroad," he added.
But the Philippines' Justice Secretary Menardo Guevarra defended the immigration bureau's refusal to allow Filibeck into the country.
"It is unlawful for aliens staying in our country to engage in partisan political activities, and the government has the right to refuse entry to those who have committed these illegal acts in the past," Guevarra said in a text message to journalists.
Read more:Duterte's deadly crackdown has sparked international condemnation.
Thousands deadAt least 4,100 people have been killed resisting arrest during the police's anti-narcotics operations. Some 2,300 more people have died in what could be drug-related killings by masked gunman on motorcycles since the summer of 2016.
Akbayan officials said President Duterte, who is under scrutiny from the international community, including the International Criminal Court, for the killings "is scared of the global condemnation against his policy to kill."
Akbayan lawmaker Tom Villarin vowed a parliamentary investigation into "this shameful incident."
"The ridiculous charge of illegal political activities is Duterte's way of saying he wants to criminalize criticisms," Villarin said.
Read more: Duterte slashes budget for human rights commission
bik/tj (Reuters, dpa)
Each evening at 1830 UTC, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.