Departing from his text, at the celebration of the Food and Agriculture Organisation's 60th anniversary, he accused Mr Bush and Mr Blair of illegally invading Iraq and looking to unseat other governments.
"Must we allow these men, the two unholy men of our millennium, who in the same way as Hitler and Mussolini formed [an] unholy alliance, formed an alliance to attack an innocent country?" he said.
"Is this the world we desire? The world of giants and international terrorists who use their state muscle in order to intimidate us? We become the midgets," he said.
He concluded his speech by thanking FAO secretary general Jacques Diouf for inviting him despite protests by the US .
While he received applause from some delegates in Rome, his speech was condemned by Mr Blair. "Nothing that Mr Mugabe says surprises us or will deflect us from our view of what is going on in Zimbabwe, which is far from a laughing matter," Mr Blair's spokesman said.
Venezuela's President Hugo Chavez turned his speech into a tirade against "the north American empire" which is threatening "all life on the planet" through pollution and climate-change problems.
Tony Hall, the US ambassador to UN food agencies, said: "These leaders chose to politicise an event that was meant to be about feeding the hungry people of the world. Mugabe, especially, should not have been invited. He would be the last person, I think, an organisation should invite to talk about hunger."
Zimbabwe is struggling to feed an estimated 3.8 million people, and has to import at least 37,000 tons of maize a week. Aid agencies have estimated that up to five million of Zimbabwe's 12 million people will need food aid by the end of the year. The seizure of white-owned commercial farms and prolonged drought have crippled Zimbabwe's economy. But Mr Mugabe's speech, a day after World Food Day, defended the land reforms as "redressing the past gross imbalances in land ownership which were institutionalised by British colonialism".
"Countries such as the US and Britain have taken it upon themselves to decide for us in the developing world, even to interfere in our domestic affairs and to bring about what they call regime change," he said.
The EU imposed a travel ban on Mr Mugabe after accusations of vote rigging. But he is allowed to attend UN events in Europe.Reuse content