The Brazilian president, who has spoken previously of his admiration for the military dictatorship that ruled his nation from 1964 to 1985, praised former Paraguayan dictator Alfredo Stroessner as a "man of vision." The ex-Army capitain also hailed Brazilian general João Batista Figueiredo for inaugurating the dam.
Speaking at a ceremony alongside Paraguay President Mario Abdo Benítez –whose father served as Stroessner's private secretary for 25 years – Bolsonaro eulogised a list of Brazilian military dictators starting with Humberto de Alencar Castelo Branco (1964-1967), Artur da Costa e Silva (1967-69), Emílio Garrastazu Médici (1969-74) and Ernesto Geisel (1974-79).
Figueiredo, who served from 1979-85, was the dictator who restored democracy when he stood down as president.
Stroessner served as Paraguay president for 35 years following an Army coup, until his death in 1989.
The Itaipu Dam, located on the Paraná river on the border between Brazil and Paraguay, is one of the two hydroelectricity plants that produce the most power in the world, alongside China's Three Gorges Dam. Construction of the project, one of the world's biggest, was completed in 1984 during Stroessner's rule. It is jointly administered by the two nations.
"All of this was possible because a man of vision, a statesman who knew perfectly well that his country, Paraguay, would need to grow. So here is my tribute to honour general Alfredo Stroessner," Brazil's president said, speaking on the Paraguayan side of the dam.
In 1995, Paraguay's Congress passed a bill calling Stroessner's 35-year rule a dictatorship. Victims of human rights violations were allowed to sue for damages. Stroessner was overthrown by a coup in February 1989 and went into exile in Brazil's capital, where he died on August 16, 2006, at age 93.
Paraguayan political analyst Ignacio Martínez said Bolsonaro likely praised Stroessner during his visit either to thank Abdo Benítez, "knowing that he admired the dictator because his father was Stroessner's private secretary, or because Bolsonaro has a military mentality, verticalist, that was formed by command structures."
Besides waxing nostalgic for Brazil's dictatorship on the campaign trail, after taking office Bolsonaro named former military personnel to seven of his Cabinet's 22 ministerial posts.
At the end of his speech, Bolsonaro told Paraguay's president: "It will be a pleasure to welcome you in Brasilia where we will deepen discussions about the welfare of our peoples. The left never again."
Brazilians have a positive impression of Bolsonaro but not two of his signature policies, according to a poll published on Tuesday.
Bolsonaro scored 57.5 percent in approval for his "personal performance" in the poll by the MDA institute on behalf of the National Transport Federation. It was the highest score for a president since November 2013, when leftist Dilma Rousseff was in power.
Bolsonaro's government only received a 38.9 percent approval rating while 56.8 percent of the more than 2,000 people polled felt his sons "are interfering in the father's decisions as president of the republic."
The pension reform presented by Economy Minister Paulo Guedes was seen negatively by 45.6 percent compared to just 43.4 percent positive replies.
Setting the minimum retirement age for men at 65 and 62 for women, it required Brazilians to have worked 40 years to receive the full amount, compared to the previous 30 years for women and 35 for men, with no minimum age.