He has an authoritarian streak a mile wide, say his foes, and has ruled his country longer than Robert Mugabe. Yet José Eduardo dos Santos is the African autocrat that perhaps the world knows least about.

Despite having never been formally elected, Dos Santos has been in power in Angola for 33 years, making him Africa's second longest-serving head of state – just behind Equatorial Guinea's Teodoro Obiang Nguema Mbasogo.

He is widely expected to remain president after Friday's election and, following a constitutional change, is well positioned to continue to at least 2022. Yet despite allegations of corruption and human rights abuses, he will gain only a fraction of the opprobrium afforded Mugabe and others.

"The world screams when a president changes the consitution for a third term," said Rafael Marques de Morais, an anti-graft activist and journalist. "This guy has been in power for 33 years and he'll be there for 43 years but the world has been silent."

Critics liken Dos Santos to a Machiavelli who has cleverly hidden in plain sight, eschewing the inflammatory rhetoric of his counterparts while shrewdly courting the US, Brazil and China. The picture of his face on campaign posters around Angola is mild, understated, almost benevolent.

De Morais continued: "He is a dictator who is not as flamboyant as the ordinary African dictators and this has misguided many people in Africa. Mobutu Sese Seko [of Zaire/ Democratic Republic of the Congo] was flamboyant and loved to talk to the media.

"How many interviews with Dos Santos can you find? Hardly any. I've never seen a picture of him sitting at his desk working. We all know the White House in America but after 33 years people have no idea what our president's office looks like. In fact very little is known about him."

De Morais, who has published several reports alleging the involvement of Dos Santos's family and political allies in corrupt business deals, added: "He is a withdrawn dictator, but the way he uses businesses to benefit his family puts Mobutu to shame. He passes laws to do it. He has always relied more on corruption than any other dictator. Obiang would not co-opt the opposition into the security apparatus as he has."

Sitting at his laptop at home, De Morais pulled up what he says is a pre-election audit of Friday's poll by Deloitte, revealing irregularities such as late voter registration and an illegal lack of transparency over polling stations. "The whole system is just fraudulent."

The son of a bricklayer, Dos Santos joined the Popular Movement for the Liberation of Angola (MPLA) as a teenager and rose quickly up the party ranks during Angola's fight for independence from Portugal. In 1979, after the death of liberation president Agostinho Neto from cancer, he was sworn in as president.

He has an iron grip on all aspects of government, heads the armed forces and appoints senior judges. He is never criticised by the country's state media and has swiftly crushed dissent in the few privately owned newspapers.

Elias Isaac, country director for the Open Society Initiative of Southern Africa, said: "Mussolini and Pinochet and Stalin and Bokassa and Idi Amin were dictators. I wouldn't classify Dos Santos as a dictator; he respects some differences and allows the existence of other political parties.

"But he is autocratic. He has concentrated so much power around himself. It's so Machiavellian, the way he has played it. Up to 1992 the MPLA was so strong. He gradually weakened the party and became strong himself. He's now stronger than the party.

"He disabled personal enemies – generals, police, politicians – by giving them diamonds and businesses and wealth. They say, 'We don't want to disturb this.' He has created a system of blood sucking and he is the main vein. They cannot let him go. A blood sucker will not survive if the main vein is not there."

Dos Santos, a natty dresser who turned 70 this week, is married to former flight attendant Ana Paula dos Santos, 18 years his junior, and has several children. The family controls a huge business empire. His eldest daughter, Isabel, is a powerful businesswoman in Portugal with shares in energy, telecoms and financial companies.

Rui Pinto Andrade, an MPLA spokesman, described the president as a nature lover who enjoys playing basketball and football. He dismissed the charge of an opaque dictatorship. "If the people voted for him, what is the problem with that? Do you think a dictator is given so much respect when he goes to places like Europe and America? Here you can see he is loved by the people."