Brazil is on the verge of electing an extremist “barbarian” who represents “the dross of the dictatorship”, his leftwing rival for the leadership of the world’s fourth biggest democracy has warned.

The far-right populist Jair Bolsonaro and the leftist Fernando Haddad will go head-to-head on Sunday in the decisive second-round of Brazil’s presidential election, with polls giving Bolsonaro an 18-point lead over the Workers’ party (PT) candidate.

On Tuesday, in one of his last major interviews before the vote, Haddad intensified his bid to claw back some of that ground, slamming his opponent as a real and present danger to Brazil’s young democracy.

“We are dealing with a barbarian, from the democratic point of view,” Haddad told journalists in Rio de Janeiro.

Haddad claimed Bolsonaro – a former paratrooper known for his admiration of the military regime that ruled Brazil from 1964 until 1985 – represented “the dross of the dictatorship”.

Quick Guide Brazil's dictatorship 1964-1985 Show How did it began? Brazil’s leftist president, João Goulart, was toppled in a coup in April 1964. General Humberto Castelo Branco became leader, political parties were banned, and the country was plunged into 21 years of military rule. The repression intensified under Castelo Branco’s hardline successor, Artur da Costa e Silva, who took power in 1967. He was responsible for a notorious decree called AI-5 that gave him wide ranging dictatorial powers and kicked off the so-called “anos de chumbo” (years of lead), a bleak period of tyranny and violence which would last until 1974. What happened during the dictatorship? Supporters of Brazil’s 1964-1985 military regime - including Jair Bolsonaro - credit it with bringing security and stability to the South American country and masterminding a decade-long economic “miracle”. It also pushed ahead with several pharaonic infrastructure projects including the still unfinished Trans-Amazonian highway and the eight-mile bridge across Rio’s Guanabara bay. But the regime, while less notoriously violent than those in Argentina and Chile, was also responsible for murdering or killing hundreds of its opponents and imprisoning thousands more. Among those jailed and tortured were Brazil’s first female president, Dilma Rousseff, then a leftwing rebel. It was also a period of severe censorship. Some of Brazil’s best-loved musicians - including Gilberto Gil, Chico Buarque and Caetano Veloso - went into exile in Europe, writing songs about their enforced departures. How did it end? Political exiles began returning to Brazil in 1979 after an amnesty law was passed that began to pave the way for the return of democracy. But the pro-democracy “Diretas Já” (Direct elections now!) movement only hit its stride in 1984 with a series of vast and historic street rallies in cities such as Rio de Janeiro, São Paulo and Belo Horizonte. Civilian rule returned the following year and a new constitution was introduced in 1988. The following year Brazil held its first direct presidential election in nearly three decades.

“[He] is putting at risk everything that has been built over the last 30 years,” Haddad said of Bolsonaro, who has also voiced admiration for Latin American dictators including Alberto Fujimori and Chile’s Augusto Pinochet.

Questioned over whether it was accurate to describe his rival a “fascist”, Haddad replied: “Forgive me, but I have the right, as a political scientist, to tell the Brazilian nation what he is.”

“He has a torturer as his idol. For me this is fascism. I’m sorry. If you want to give him another name to sugarcoat Bolsonaro, go and find one. But I’m going to use the one I learned in the university classroom.”

Bolsonaro emerged victorious from last month’s first round, securing more than 49m votes to Haddad’s 31m, thanks to his pledges to combat crime and corruption.

Widespread public rage at Haddad’s PT – who critics accuse of leading Brazil into its worst ever recession and overseeing what some call the biggest corruption racket in world history – has also played a major role in the rise of a politician who casts himself as a Trump-style outsider despite nearly three decades as a congressman.

But the prospect of a four-year Bolsonaro presidency has horrified progressive Brazilians and those who suffered during its dictatorship, when hundreds of regime opponents were killed, tortured and disappeared.

On Sunday the populist provocateur vowed that if elected he would spearhead a historic purge of his leftwing political foes in a menacing address to supporters. “Either they go overseas, or they go to jail,” Bolsonaro said, calling opponents “red outlaws”.

He dismissed criticism of his speech as “hysteria”.