From Reuters we find more statistics from the UN study.
The number of people living in shantytowns increased by 55 million to 827.6 million as population growth and migration from the countryside outstripped the effect of upward mobility in cities, the U.N.'s biennial report on cities found.
"The situation has improved over 10 years, but alas over the same period, the net increase of the urban poor is 55 million," Anna Tibaijuka, the executive director of the U.N. Habitat program, said in Rio de Janeiro.
The Brazilian city will next week host the World Urban Forum, a five-day U.N. conference on the state of the world's cities, where more than half the global population now lives.
Some 227 million people escaped slum conditions from 2000 to 2010, meaning that countries easily surpassed their collective target under the U.N. Millennium Development target, the report said.
Tibaijuka played down the achievement of beating the Millennium goal of pulling 100 million people out of poverty, calling it "totally inadequate." The Millennium goals include cutting extreme poverty, reducing child mortality and fighting epidemics by 2015.
Barring "drastic" action, the number of slum dwellers in the world's cities is expected to grow by 6 million a year over the next decade to hit 889 million by 2020, the report said.