Slums and high-rises in Rio de Janeiro, Brazil / Christian Science Monitor

"If a city has no structure, there will be inequality," said Joan Clos, head of the UN Human Settlements Program (UN-Habitat) during the opening days of the World Urban Forum in Medellin, Colombia. Clos, the former mayor of Barcelona, said far too often cities in the developing world appear to have been built with a total disregard for organizational structure. These cities have set themselves up for massive social unrest.

Across the developing (and developed) worlds, there has been "metastatic growth," much like a cancerous tumor eating its way through healthy tissue. This is because "urban growth has been developer-driven. High-rises and shopping malls are placed at random, creating disaggregation and then segregation." This segregation happens because "low-income people are absent in developers' considerations. Developers don't make any money off of them."

To avoid social unrest in increasingly unequal cities, Clos said "we need to start planning public spaces again." Developers must learn to work within the frameworks set by planners. He added that pretty master plans are not urban planning. "If the first thing you see from a developer are renderings with all the houses filled in, then there has been no public input."

Looking at the state of global urbanization — 80 percent of the world will be living in cities by 2050 — Clos is "flabbergasted." He sees all the social splintering and fragmentation that is to come if there isn't a new global investment in fair planning.

Echoing his remarks, a number of urban leaders from around the world explained how they are working towards more socially-cohesive cities.

Rossyln Greef with the city of Johannesburg, South Africa, showed a wowing video of their new "corridors of freedom" initiative, which aims to create a new spatial plan that will undo the segregation built into the city with apartheid.

Johannesburg's corridors of freedom are really high-density, mixed-use developments along bus-rapid transit (BRT) corridors. The city's goal is to reduce commuting costs for the city's poor, so they are targeting those areas first.

She said: "We are moving from a deliberately exclusionary framework to an inclusive one. We are getting rid of racial segregation through planning. It's a huge challenge."

Ali Mandanipour, a professor at Newcastle University in the UK, pointed to the 2006 strategic plan from the city of Antwerp in Belgium as a model for how to reconnect a city and envisage a more equitable future. He also highlighted Bogota, Colombia's huge investment in the planning and design of public spaces, which are all wheelchair accessible.

Mandanipour said "public spaces are key breathing spaces that make cities more attractive for people and investors." However, he also cautioned, that new parks and plazas can become a tool for gentrification and exclusion if their construction pushes the poor out. "Spatial linkages must connect with existing social linkages."

The United States has had a long history of segregation and social inequality, said Lisa Rice, with the National Fair Housing Alliance. She said the fair housing laws passed in the wake of the assassination of Martin Luther King, Jr. helped eliminate housing discrimination and advanced social cohesion but segregation persists. Detroit, Washington, D.C. and Chicago are all "hyper-segregated," which fuels disparities in access to education, healthcare, transportation, and food. "Detroit has huge food deserts. There are no supermarkets within the city limits. You have to drive to the suburbs." In the U.S., her organization and others are trying to "stave off predatory lending in low-income areas."

For the minister in charge of urban development in Argentina, Daniel Chain, the words of Pope Francis are worth heeding; "If a society abandons parts of itself, it will have no peace of mind. Inequality leads to violence because, at its essence, it's unfair." In Buenos Aires, Chain helped undo the damage caused by the red-lining that occurred in neighborhoods near one highway, helping to bring them back to life. The city has also undergone an intensive program of building new theatres, schools, libraries in its southern areas, its poorest sections. Chain said "poor people who live right next to the wealthy receive a slap in the face, a blow to their dignity." It's a "true confrontation" Buenos Aires is trying to limit.

And Jean-Marie Kazadi, head of urban development for the Katanga province in the Democratic Republic of Congo (DRC) talked about the immense challenges in starting to plan communities where there has been no planning to date due to persistent civil war. Beyond war, the cities of Katanga must deal with the legacy of the harsh Belgian colonialism, which, he said, the DRC government just perpetuated after independence. "We must keep people at the center of our concerns."