The number of city dwellers is at an all-time high of about 3.5 billion and will nearly double in the next 30 to 40 years, with almost all the growth in developing countries, the head of the UN agency focusing on cities said. Joan Clos said even though the rate of population growth is decreasing, the UN estimates that in the next 30 years the global population will increase from 7 billion to 9 billion - and the urban population will grow by between 2.5 billion and 3 billion. "In all human history we have reached 3.5 billion of urban settlers and in the next 30 years we are going to have 3 billion more," Clos said. "Imagine the changing rate - what we have done in all human history, we nearly will do in the next 30 to 40 years." With 96 per cent of the growth of cities expected in poorer developing countries, he said, there were going to be huge demands on land, resources and services. Clos, a former mayor of Barcelona, is executive director of the UN Human Settlements Programme, known as UN-Habitat. He was speaking at a news conference promoting the agency's World Urban Forum from April 5 to 11 in Medellin, Colombia, which will focus on growing inequalities in urbanisation worldwide. He said 10,000 participants were expected, including ministers, mayors, academics and representatives from business, non-governmental organisations and local authorities. Currently, Clos said, the world was experiencing "the highest rate of urbanisation in human history", and national and local governments didn't have the capacity to address key issues including organisation, governance, finance and the provision of services. In recent decades, he said, inequalities in urban areas have led to protests and unrest as cities have faced difficulties integrating a large influx of migrants. UN-Habitat said it estimates that the world's slum population rose from 650 million in 1990 to an estimated 863 million last year. The challenge is whether the growth of cities can be carried out "in a planned and designed manner, in order to provide some basic services at affordable costs for the citizen", Clos said. Otherwise, they would grow spontaneously without any planning.