ROME (AFP) - UN chief Ban Ki-moon called for a huge rise in food production Tuesday as world leaders opened a summit on the food price crisis that threatens hunger, poverty and conflict worldwide. In talks clouded by controversy focused on the presidents of Zimbabwe and Iran, the UN Secretary-General said food output had to rise 50 per cent by 2030 to meet rising demand. "We have a historic opportunity to revitalise agriculture," Ban told some 50 heads of state and government, gathered for the three-day summit. "I call on you to take bold and urgent steps to address the root causes of this global food crisis," he said at the Food and Agriculture Organisation headquarters in Rome. With food prices at a 30-year high, the UN Secretary-General warned that while the world must "respond immediately," it must also put the long-term focus on "improving food security." Prices have doubled in three years, according to the World Bank, sparking riots in Egypt and Haiti and in many African nations. Brazil, Vietnam, India and Egypt have all imposed food export restrictions. Rising use of biofuels, trade restrictions, increased demand from Asia to serve changing diets, poor harvests and increasing transport costs have all been blamed for the price rise. World Bank President Robert Zoellick has said two billion people across the world are struggling with high food prices, and 100 million extra people in poor countries may be pushed into poverty by the crisis. The UN chief said it was also essential for the Doha round of World Trade Organisation talks to be completed as quickly as possible to alleviate the crisis. UN agencies have launched appeals for more than one billion dollars to alleviate the food crisis. Saudi Arabia has already given 500 million dollars to a World Food Programme appeal. Estimates vary on the extent to which demand for biofuels has pushed up food prices, ranging from 30 percent by some experts to less than three percent according to the US Agriculture Department. Brazilian President Luiz Inacio Lula da Silva defended biofuels, saying they could be an "important tool" against food insecurity. "Biofuels are not bandits ... We must remove the smokescreen of powerful lobbies that blame ethanol production for the rise in food prices. It's a mockery, an affront," he said. Japanese Prime Minister Yasuo Fukuda urged fellow leaders to release excess stockpiles of food to ease shortages in poorer countries, offering more than 300,000 tonnes of imported rice held by Japan. The president of Senegal, Abdoulaye Wade, voiced disappointment with the UN food body. "We can't continue to be helped like beggars," he said. "I have been disappointed ... Don't keep imposing institutions (and) experts on us. Africa is not what it was 20 years ago. Stop this farce." The summit opened amid controversy over the presence of Zimbabwean President Robert Mugabe, whose attendance at the talks was described as "obscene" by Britain's International Development Minister Douglas Alexander. Mugabe defied the criticism when he took the podium at the Rome meeting, claiming that Britain was seeking to "cripple Zimbabwe's economy and thereby effect illegal regime change in our country."