In his first speech, Milanovic called on all sides in Croatia to work together in combating corruption, which he said affected every area of life, as well as efforts to conceal the climate change issue, dpa said.
"It seems to me there is only one right way: full and active support by the government to academic and scientific independence and independence of the judiciary and media," it quoted him as saying.
"The scientific community, the judiciary and the media must work incessantly on perfecting the mechanisms for fighting against dishonesty and corruption in their own ranks," Milanovic added per dpa.
The agency said Milanovic was thought to have a weak chance of defeating then incumbent president Kolinda Grabar-Kitarovic in the first round of the vote on December 22. However, he used her mistakes and a fall in the popularity of the ruling HDZ party, which backed Grabar-Kitarovic, winning in both rounds.
Reuters reported that Milanovic urged more solidarity in society in his inauguration speech and called for vigilance against populist approaches to issues such as climate change that might distort or ignore scientific facts.
Strengthening solidarity in society, a more just distribution of wealth, and a fight against clientelism, the culture of lies and greed are the main and the most effective tools against inequality and further class and human disintegration and alienation, Milanovic said.
Unlike his four predecessors who were sworn in at Zagreb’s mediaeval St. Mark Square that houses the government and parliament buildings, Milanovic, 53, chose to organise a smaller ceremony at his office on a hill in a largely residential area overlooking central Zagreb, Reuters said.
"The majority of voters can vote for a decision that there are no climate changes or that it is not a problem. It won’t change the truth that the problem exists,” it quoted him as saying.
He said the truth was one thing and another thing was "a legitimate political will to do something with that truth, or do nothing," Reuters reported.
"Independent media, the judiciary and the scientific community are the strongest defense against any form of tyranny," said Milanovic, who was prime minister from 2011 to 2015, the agency added.
"I will not be a corrective, but a constructive (political) factor," it quoted Milanovic as saying.