While also condemning the attack, the conservative opposition New Democracy, which is leading Mr. Tsipras’s leftist party, Syriza, in opinion polls, responded by accusing the prime minister of the “political and moral legitimization of violence” through “polarizing and toxic rhetoric.”
Members of far-right groups, however, could barely contain their excitement. “Bravo to each and every one who carried out his duty in Thessaloniki today. Respect,” wrote Ourania Michaloliakou, the daughter of Nikos Michaloliakos, the leader of Golden Dawn, on Twitter. In another, she wrote, “My God, why can’t I be in Thessaloniki today,” adding, “Well done, dudes.”
Christos Paschalidis, an official with a nationalist group, Anasa, in the northern city of Kavala, struck a similar note with a post on Facebook, saying that Mr. Boutaris “got what he deserves. Well done to the boys.”
“Now it’s the turn of the traitor” lawmakers, he added, using a term often used by populists to condemn politicians who backed Greece’s acceptance of international bailouts that imposed years of austerity on the country.
The assault was the focus of a front-page story by a Turkish newspaper, Hurriyet, with the headline, “An attack on friendship.” That was an apparent reference to Mr. Boutaris’s championing of closer ties between Greece and its traditional rival, Turkey, and efforts to increase tourism between the two countries.
Mr. Boutaris has hailed Thessaloniki as the birthplace of Mustafa Kemal Ataturk, the progressive founder of modern Turkey, and he has been instrumental in pushing plans for the creation of a Holocaust museum in the city, which lost some 50,000 Jews to the Nazis.
In March, statistics from the Greek police showed that hate crimes more than doubled last year, with 184 attacks recorded, compared with 84 in 2016. Some observers have said that most assaults go unreported and that the actual figures are much higher.
Mr. Boutaris was scheduled to speak at an emergency session at Thessaloniki’s City Hall on Monday afternoon, and a solidarity rally has been organized for Wednesday.