The careers of two of Africa's most prominent politicians - Robert Mugabe and Nelson Mandela - have striking similarities. Both were born in an era when white power prevailed throughout Africa, Mandela in 1918, Mugabe in 1924. Both were products of the Christian mission school system, Mandela of the Methodist variety, Mugabe of the Catholic. Both attended the same university, Fort Hare in South Africa. Both emerged as members of the small African professional elite, Mandela a lawyer, Mugabe a teacher. Both were drawn into the struggle against white minority rule, Mandela in South Africa, Mugabe in neighbouring Rhodesia. Both advocated violence to bring down white-run regimes. Both endured long terms of imprisonment, Mandela, 27 years, Mugabe, 11 years. Both suffered the anguish of losing a son while in prison; and both were refused permission to attend the funeral.

But whereas Mandela used his prison years to open a dialogue with South Africa's white rulers in order to defeat apartheid, Mugabe emerged from prison bent on revolution, determined to overthrow white society by force. Military victory, said Mugabe, would be the "ultimate joy".

Even after seven years of civil war in Rhodesia in which at least 30,000 people died, when the opportunity to gain power through elections was on offer, Mugabe expressed his disappointment that he would be denied the kind of power that military victory would have given him. Power for Mugabe was not the means to an end but the end itself.

This year Mandela celebrates his 90th birthday, acclaimed around the world as one of the great leaders of his time, while Mugabe battles on grimly after 28 years of power in Zimbabwe like an 18th-century prizefighter blinded by his own blood - and the blood of many others.

Yet the early years of Mugabe's rule seemed so full of promise. Instead of the angry Marxist ogre that the white minority had feared, after winning the 1980 election Mugabe appeared as a model of moderation, pledging to work for reconciliation and racial harmony. Even the recalcitrant white leader, Ian Smith, who had previously denounced him as "the apostle of Satan", now found him "sober and responsible".

Western governments lined up with offers of aid. In its first year of independence, Zimbabwe was awarded £900m in aid, enabling Mugabe to embark on ambitious programmes of education and health development. The white population, too, benefited from growing economic prosperity. Given large increases in commodity prices, white farmers - the backbone of the economy - became ardent supporters of Mugabe's government and his ruling Zanu-PF party. "Good old Bob!" they cheered.

But Mugabe's black political opponents fared less well. Within weeks of gaining power in 1980, Mugabe set out to crush political opposition in Matabeleland and establish a one-party state. The military campaign he unleashed there in the 1980s culminated in mass murder. As many as 20,000 civilians are estimated to have died. But it gave Mugabe the total control he had always sought.

In Harare, meanwhile, Mugabe's inner circle scrambled for property, farms, businesses and government contracts. Mugabe joined the fray, but his real obsession was not with personal wealth but with power. Year by year, he acquired ever greater powers, ruling the country through a vast system of patronage, favouring loyal aides and cronies with government positions and ignoring the spreading blight of corruption. "I am rich because I belong to Zanu-PF," boasted one of Mugabe's proteges, a millionaire businessman. "If you want to be rich you must join Zanu-PF."

With his one-party system, Mugabe's tentacles reached into every corner of the land. One by one, parliament, the state media, parastatal organisations, the police, the civil service and, eventually, the courts, were subordinated to his will. In dealing with dissidents, his secret police were licensed to harass, intimidate and even murder at will.

By the mid-1990s, Mugabe had become an irascible and petulant dictator, brooking no opposition, contemptuous of the law and human rights, surrounded by sycophantic ministers and indifferent to the incompetence and corruption around him. Whatever good intentions he had started out with had long since faded. A land reform programme financed by Britain came to a halt when it was discovered that Mugabe was handing out farms intended for peasant resettlement to his own cronies.

Ordinary people meanwhile suffered the brunt of government mismanagement. By 2000, Zimbabweans were generally worse off than they had been at independence: average wages were lower; unemployment had trebled; public services were crumbling and life expectancy was falling.

As opposition to his rule mounted, Mugabe struck back with increasing ruthlessness. His first target was white farmers who, worried about their title to land, had shown signs of supporting a new opposition coalition, the Movement for Democratic Change. Hoping to bolster his popularity, Mugabe sent gangs of party activists to rural areas to seize control of white-owned farms to distribute to his supporters, but it led only to the collapse of the agricultural industry.

His ultimate objective, however, was to crush all opposition and remain in power for as long as he wanted. Since 2000, he has used all the government's resources to attack his opponents, sanctioning murder, torture and lawlessness of every kind, rigging elections, violating the courts and suppressing the independent press. In a speech in 2003, he warned he would use even worse violence if necessary, threatening to act like a "black Hitler" against the opposition. "If that is Hitler, then let me be a Hitler tenfold. That is what we stand for."

The cost of this strategy has been enormous. Zimbabwe has been reduced to a bankrupt and impoverished state, threatened by economic collapse and catastrophic food shortages.

But still Mugabe fights on. "No matter what force you have, this is my territory and that which is mine I cling [to] unto death," he said in 2001. And he is far from finished. Though losing control of parliament in last month's election, he can still rely on party militias, youth groups, war veterans, police and army generals to help him win the next round of the presidential election. Violence has been Mugabe's stock-in-trade for more than 30 years. Indeed he has boasted that he has "a degree in violence". It is not a pleasant prospect for Zimbabweans yearning for something better.

· Martin Meredith has written biographies of both Robert Mugabe and Nelson Mandela. He is the author of The State of Africa: A History of Fifty Years of Independence. His latest book is Diamonds, Gold War: The Making of South Africa

comment@theguardian.com