A number of children were killed in the incident, which unfolded at a shopping centre that hosted a fireworks display.
One eyewitness explained that people – including his friend – suffocated after getting caught between crowds that were moving in opposing directions.
Big gatherings for New Year’s Eve were allowed for the first time since the start of the Covid-19 pandemic.
Kampala police said people had attended a concert at Freedom City Mall in large numbers.
A crowd surge was said to occur when partygoers tried to head back inside – having gone outdoors to see the midnight fireworks.
Five people – described as “mostly children, juveniles” – were said to have been killed after being stepped on.
The death toll was later put at nine after a number of people were taken to hospital.
An eyewitness, Ramadhan Aphongo, told local broadcaster NTV that a friend was among the victims: “She was enjoying, she was having fun, but due to loss of breath… eventually she died.”
An investigation is underway.