Serious flooding began on Sunday in parts of northern Mozambique that were hit by Cyclone Kenneth three days ago, with waters waist-high in places, after the government urged many people to immediately seek higher ground.
Hundreds of thousands of people were at risk.
"Help us, we are losing everything!" residents in the region's main city, Pemba, shouted at passing cars as the rushing waters flooded their homes and heavy rain fell.
Women and girls used buckets and pots to try to scoop the torrent from their front yards, in vain. The water poured into doorways.
Houses began to collapse and at least one rescue team was mobilized, United Nations staffers said.
"We are unfortunately expecting devastating floods," the U.N. humanitarian agency said in a tweet.
Some Pemba residents tried to pile up tires and sand-filled sacks as barricades. Children took refuge in a bus that appeared to be stuck as vehicles struggled on the streets.
One woman stood in front of the bus, seemingly stunned, as the rain pounded down. Cars began to slip under the waters.
"We will keep moving until we get somewhere safe," one man said, as people fled carrying their belongings in plastic bags. "I have never seen such rains in my life," said another resident, Michael Fernando.
Authorities have said at least five people died after Cyclone Kenneth arrived Thursday evening with the force of a Category 4 hurricane, stunning residents of a region where such a storm had not been recorded in the modern era.
Kenneth came just six weeks after Cyclone Idai ripped into central Mozambique and killed more than 600 people.
The remnants of Kenneth could dump twice as much rain as Idai did, the U.N. World Program has said.
It was the flooding after Idai that caused most of the deaths. As much as 250 millimetres of torrential rain, or about a quarter of the average annual rainfall for the region, is forecast over the next few days.
This was the first time in recorded history that the southern African nation has been hit by two cyclones in one season, again raising concerns about climate change.
Nearly 700,000 people could be at risk in the largely rural region, many already exposed and hungry. Some rivers in the region have burst their banks in the past, notably in 2000.
There was no immediate word on Sunday of other districts that had been hit much harder by the cyclone.
Aerial photos taken on Saturday showed several coastal communities flattened by the storm in Mozambique's northernmost Cabo Delgado province.
"Not a single house is standing anymore," Saviano Abreu, a spokesman with the U.N. humanitarian agency, told reporters after the aerial assessment.
With about 3,500 homes in parts of Cabo Delgado partially or fully destroyed, families waded to what they hoped were safer areas or huddled under impromptu shelters.
Already, livelihoods have been lost and people are wondering how they will cope in a country struggling with one of the world's highest poverty rates.
With notebook and pen in hand, elderly Luis Momade walked near the beach in Pemba on Saturday, taking advantage of a lull in the rains to quantify the damage from the cyclone.
The president of the local Paquite Residents Association, his notebook was almost full with names and figures of boats damaged or destroyed.
With unemployment rife and many in coastal areas surviving with fishing and related activities, not going to sea could mean going hungry for days.
Men, women and children foraged in the waters off the littered shore, looking for seashells to sell.
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