Most of humanity will be living in cities by next year, raising the threat of increased poverty and religious extremism unless the needs of growing urban populations are met, the U.N. said Wednesday.

Some 3.3 billion people will live in cities by 2008, a report by the U.N. population agency report said. By 2030, the number of city dwellers is expected to climb to 5 billion.

Without proper planning, cities across the globe face the threat of overwhelming poverty and limited opportunities for youth, said U.N. Population Fund Executive Director Thoraya Ahmed Obaid.

"In 2008, half of the world's population will be in urban areas, and we are not ready for them," said Obaid told The Associated Press in London.

A revival in religious interest has been a surprising characteristic of rapid urbanization, according to the report.

Urbanization is often associated with a shift toward secular values. But the growth of new religious movements — such as radical Islam in the Middle East, Pentecostal Christianity in Latin America and the cult of Shivaji in India — has been a primarily urban phenomena, the report said.

When cities fail to meet the needs of growing populations, religious beliefs tend to become extreme, said Obaid, who is also a U.N. undersecretary-general.

"Extremism is often a reaction to rapid and sudden change or to a feeling of exclusion and injustice, and the cities can be a basis for that if they are not well managed," Obaid said.

Smaller cities will absorb the bulk of urban growth, the report said.

"We're focusing on the megacities when the data tell us most of the movement will be coming to smaller cities of 500,000 or more," Obaid said.

Smaller cities may be more flexible in expanding their boundaries and adapting their policies, but they also have fewer resources and smaller governments than major cities accustomed to large migrant populations.

The population fund found that policy initiatives in smaller cities often aim to keep the poor out by limiting migration and cutting lower-income housing.

"Cities see poor people as a burden," Obaid said. "They should be seen as an asset."

"Investing in them in terms of shelter, education and so on would mean you have a good economic force that can work and create even further economic growth coming from cities," Obaid said.

Birth rates are driving urban population growth, rather than migration from rural areas, the report said. Family planning policies will be most effective in slowing that expansion, including reproductive health services and sex education, it said.

"Urban growth, in a sense, encourages low fertility because city people have access to information and access to services and can plan their families better," Obaid said. "In an urban economy, women need less children but (want children) with a better quality of life and better possibilities of education."