The Disasters Emergency Committee aims to get urgent supplies of food to eight million people whose harvest was destroyed last year by droughts and locusts in Niger and neighbouring countries in the Sahel region of West Africa.
The Red Cross Federation, meanwhile, will begin distributing food to 23,000 children and their families in four of the worst hit parts of the country today.
Mark Snelling, part of the British Red Cross operation in Niger, said: "There are centres for the severely malnourished, but we also want to provide food for those who are not yet in need of urgent medical attention. If we feed them now, we hope to stop the situation getting out of hand."
The UN's World Food Programme and British charities such as Save the Children have already flown vital supplies to Niger, but aid agencies in the field say demand for more supplies is still a high. They also hope that by getting food to countries such as Burkina Faso, Mali and Mauritania, they can save people there from Niger's plight.
The DEC, an umbrella group which brings in charities such as Action Aid and Oxfam will run a radio and TV campaign urging people to make telephone and on-line donations straight away.
"We need the public to donate whatever they can today to help us save lives," said Brendan Gormley, the DEC's chief executive. "DEC members are working together to get vital supplies of food to those who need it most urgently in Niger and are also scaling up their efforts in the other countries of the Sahel region in West Africa, which are also badly affected. Every second counts."
Meanwhile, Médecins Sans Frontières, the aid agency that is treating the most acutely malnourished people in Niger, warned that a similar crisis has hit south Sudan. The area has suffered from a poor harvest and erratic rains, and food supplies have been further depleted by tens of thousands of refugees who have returned there after the end of a 20-year civil war.
In just one area, Aweil East county, 7,000 children are close to starving to death.
The current spate of food shortages across Africa prompted the Secretary of State for International Development, Hillary Benn, to call for a humanitarian fund which would give the UN the ability to respond to threats of famine without waiting for money from the international community.
"Does the international system work properly in these terrible tragedies? The answer is, it doesn't," he told the BBC. "The crisis we are now seeing unfolding in Niger is a really good reason why we have to do better in the future."
The UN and the government of Niger warned last November that a plague of locusts and a series of droughts had created acute food shortages in parts of the country, but the international community did not raise funds until this May.