Save the Children said the most vulnerable faced starvation unless they get food aid in the next couple of weeks.
"The rising malnutrition and the rise in diseases are going to mean that children will die and we have to act very fast," said Sarah Jacobs, a spokeswoman for the relief group.
The United Nations had said previously that more than 5 million people in Zimbabwe would need food aid by early next year after a poor harvest compounded by economic turmoil.
Jacobs said many people in the Zambezi Valley, the poorest and driest area, were now surviving on a vile-tasting, fibrous root called makuri.
"It's got no nutritional value whatsoever. It tastes disgusting and it also has a parasite which attaches to it which is toxic," said Jacobs, who has just returned from the region.
"This is all they have to eat. You see babies eating it and toddlers eating it, and it's not digestible. It creates terrible stomach pains."
People were eating anything to survive, she said. She had come across one child who had died after eating a poisonous root and young children eating tiny rats they caught in their huts. More...
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