"Those who are doing that, I don't know who they are following and on whose behalf they are doing it," he said in a public television talk show.
There have already been such outbursts and it is up to the police to find out if the three recent ones have been orchestrated ahead of the election, he added.
Plenkovic described as incorrect the Serb National Council's claims that the rise in violence was due to a benevolent stand on the Nazi-styled WWII Independent State of Croatia and the legalisation of the Ustasha salute "For the homeland ready".
"We have always clearly condemned such things, but it's difficult for the government to control individuals and people who find sense in that. We have clearly condemned all forms of hate speech."
Commenting on the fact that only one HDZ slate was headed by a woman, he said the HDZ saw to gender equality and recalled that Dubravka Suica was a member of the European Commission as well as other HDZ women in office in the EU.
Plenkovic said Homeland Movement leader Miroslav Skoro was an experiment and that Social Democratic Party president Davor Bernardic had no experience in government.
Asked with whom the HDZ would not enter a coalition after the July 5 parliamentary election, he said they would not consent to any blackmail and exclusiveness.
Asked about the most difficult period over the past four years, Plenkovic said there was not enough cohesion in the HDZ at the start but that it was achieved after intra-party elections.
"I also regret that we haven't done more about tolerance in Croatian society and inclusiveness," he said, adding that it was necessary to do that and that a country which had resolved its national issues had enough strength for it in 2020.
"I think we have enough strength, especially as victors in the Homeland War, to extend our hand," he said, adding that "we must show breadth."
Asked about the purchase of Hungarian energy group MOL's stake in its Croatian peer INA, an unfulfiled promise from his term in office, Plenkovic said the government had an estimate of INA's value and that it was a good basis for further talks.
Asked why Croatia became during his term the second poorest country in the EU in terms of GDP per capita, he said Croatia was the last to join the Union and that other member states did not have a war.
Plenkovic said his government had sound growth and that this was visible in wages. "We have strengthened the role of the state with a better and stronger position in the European Union. We never had such a position."
Asked again why Croatia became the second poorest EU member state, Plenkovic said it was not true. "We have become richer," he said, adding that the government did not have a magic wand.