Mayor Pawel Adamowicz, a critic of anti-immigrant policies, was stabbed on Sunday evening by a man who rushed the stage during a charity event. He died the following day, plunging Poland into mourning.
Polish media has reported a few incidents of threats against some liberal mayors since then.
Internal Affairs Minister Joachim Brudzinski said in statement on Wednesday: “Polish police officers throughout the country have already detained more than 10 since yesterday in connection with online calls for murders and acts of aggression.”
“Once again I appeal to stop bad emotions,” he said, promising a strong police response.
Critics have said politicians themselves were cranking up hate speech and aggression in public life, and blamed the ruling Law and Justice party for not reacting earlier.
Adamowicz was one of 11 mayors in Poland who were the targets of fake death certificates by a far-right group called All-Polish Youth after they signed a declaration to welcome refugees against the government’s anti-migration policies.
The prosecutor’s office decided not to pursue the case further.
Thousands of people have gathered in vigils in Polish cities to mourn the death of Adamowicz, who had been mayor of Gdansk for 20 years and espoused liberal causes at odds with the conservative nationalist government.
Polish authorities have arrested a 27-year-old former convict named Stefan in connection with the murder. His full name has so far been withheld.
The police said on Monday a 72-year-old Warsaw man was detained after threatening in a phone call to a social welfare center that the next in line to be killed was President Andrzej Duda.
On Wednesday Polish news agency PAP said police in Gdansk detained a 48-year-old suspected of making threats on the phone against European Council President and former Polish Prime Minister Donald Tusk.
The incidents reflect a charged political atmosphere in a number of European countries, including Poland and Hungary, where populist leaders have fanned nationalist and anti-immigrant sentiment.
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