Several cars were also washed away, along with household goods and personal items as residents moved to higher ground for safety.
The death toll in the weekend flooding triggered by heavy rains in eastern Uganda has risen to 21, the Uganda Red Cross Society (URCS) said Monday.
Esther Davinia Anyakun, Minister of State for Relief, Disaster Preparedness and Refugees, said seven of the dead were found in Mbale District, while three were found in Kapchorwa late Sunday.
URCS spokesperson Irene Nakasiita said 11 more bodies were retrieved from the River Nabiyonga on Monday morning, which brings the number of dead to 21.
"The rains have caused a lot of havoc that is beyond individual and community capacity. We ask the general public to leave all the potential waterways both in hills and valleys, landslide-prone areas; we don't want to lose more lives," the minister said.
The URCS said heavy rains had displaced thousands of people, and more than 1,000 households had been affected in neighbouring districts, while four others had been put on high alert.
The country's meteorological department warned about the northern, eastern and midwestern parts receiving heavy rainfall in August two days before the incident occurred.
On Sunday, police and the military were called in to help in the search and rescue operations in Mbale, where stranded residents could only watch helplessly as their belongings were washed away by the floodwaters.
Mbale City resident commissioner Ahamada Waashaki on Sunday told AFP that "many more people are missing and feared dead."
"There is a lot of destruction, roads cut off, buildings submerged as a result of heavy rain that started last night until this morning," he said.
He said the situation deteriorated after the Nabuyonga and Namatala rivers burst their banks, causing flooding across most parts of the city.
Prime Minister Robinah Nabbanja visited the scene of the disaster in Mbale, which lies about 300 kilometres (180 miles) northeast of the capital Kampala.
"Police and military marine forces will be coming to help in the rescue and search for dead bodies as we provide relief to the affected population," her office said.
Two local reporters told AFP they had seen bodies floating in the muddy brown floodwaters before being removed by police.
Several cars were also washed away, along with household goods and personal items as residents moved to higher ground for safety.
"In the past we experienced flooding but not the level of lives lost and destruction of property seen this time," Waashaki said.
Nabbanja suggested environmental damage caused by human activity was to blame.
"I believe this disaster would have been avoided if people did not encroach on river banks," she said in video shared by her office.