Tens of thousands of Mozambicans are stranded without food and water after floods swept through the south of the country this week, sparking a large-scale humanitarian crisis.

With the displaced now living in the open and eating grasshoppers to survive, the Mozambican government and international agencies like the Red Cross are warning of a looming catastrophe.

When the floods came on Wednesday, residents of Chokwe town escaped the raging flow of the Limpopo River with what little they could carry.

Alegria Hlangwe only had time to grab one of her children. Her other two sons were across town.

"People told me they took shelter in someone's house. I don't know if that is true," the anxious mother said.

The heaviest rains had stopped several days earlier, but it was already too late.

Swollen by downpours in neighbouring South Africa and Zimbabwe, the flood wave was on its way.

"No one really thought this could happen," said Sergio Chauke, who escaped with only his identity documents.

Those who could bundled into vehicles and fled the town.

"Those who did not have transport climbed onto the top of buildings," Chauke said.

Sergio and his sister are now among tens of thousands of people camped out by the roadside outside the town.

Kilometre after kilometre, in groups of five to 500, people huddling under trees have spread out their belongings to dry.

More are arriving constantly, some on foot along with their livestock. Most are women, children and the elderly.

They have nothing to protect them from more rains or the clouds of mosquitoes that are starting to gather.

As they wait for help to arrive, hunger is setting in.

"We are eating grasshoppers," Alice Mabunda said, but she believes they are the lucky ones.

"We have family in the town, our aunts. They must be on the rooftops now," she said.

Ten-year-old Mercia managed to save her pet rabbit from the flood but little else.

"We left at midnight without taking food. We have nothing to sleep under."

Her mother, Anastasia Antonio, is simply glad the rains have abated.

"Thank God it is not raining, but if it does it will be a crisis for the children," she said.

The risk of disease has skyrocketed.

Diarrhoea is already the biggest killer of children in Mozambique, and without latrines a cholera outbreak may not be far away.

"We dug a hole, but the ground is so soft," Antonio said. "It will only be all right for a few days, not longer. It is not safe."

A few kilometres down the road is a government camp to help flood victims.

It last operated during devastating floods of 2000 that killed an estimated 800 people and displaced millions.

But the huge crowds waiting there for food are disappointed.

"It is three days now. We have nothing to eat, nowhere to sleep, no clothes. I left as I am," said mother of six Alice Mukavele.

"There is very little food," said International Red Cross official Armando Djedji. "We are prioritising children, heads of family, the elderly and the sick."

-- 'These people have lost everything' --

So far the International Committee of the Red Cross (ICRC) is the only humanitarian organisation on the ground.

It can provide only 40 large tents -- enough for 400 people -- and is trying to install latrines and secure clean water sources.

The government had managed to bring in 55 sacks of rice, promising more was on its way.

The numbers they will have to feed are hard to estimate. Around 6,000 families had already turned up at the centre the Red Cross estimated.

"We need support here. These people have lost everything," visiting health ministry official Marco Chipanga told AFP.

Aid agencies based in the capital Maputo promised that a week's supply of food was on its way to the area late Thursday.

With their homes and belongings washed away by the floods, many now face an uncertain future.

"Nobody knows when they will be able to go back -- whether days or weeks," said Chipanga adding, "It depends if it rains any more."