The State of the World Cities report, presented at the end of the conference in the Canadian city of Vancouver on Friday, also said that 38% of urban growth will be in slums.
By 2020 six out of 10 people in the world will live in cities, the report said.
Clive Harridge, of Britain's Town and Planning Institute, said that leaving the problems of urban poverty will increase security threats.
Urban security threats are "having an increasing impact on cities, and all projections suggest urban political violence will continue to escalate".
Speakers at the conference called on governments and organisations to involve populations in the answers to solve the problems.
"We are fed up with being the subject of the agenda," said Rose Molokoane of the South African Homeless People's Federation and Slum Dwellers International.
"We are fed up with just sitting down and seeing government doing every thing for us ... give us land, give us security of tenure."
Global funding agencies need to change how they operate to improve slums, said David Satterthwaite of the UK-based International Institute for Environment and Development.
Most agencies give money to central governments "in the name of administrative convenience", he said, while adding that funds should go directly to small projects "to support the initiatives of grass-roots organisations".
The next World Urban Forum will be held in 2008 in Nanjing, China, where the pace of urbanisation is among the world's fastest.
There are 600 cities in China, compared to 220 cities 25 years ago, said Chinese researcher Xuemei Bai, of the Japan-based Institute for Global Environmental Strategies.
"This enormous transformation is not going to stop or to be reversed," she said, warning that China has to overcome its challenges of extreme pollution and the loss of agricultural areas to urban development.