London, England (CNN) -- As the world's urban population continues to grow at a rapid rate, communities around the world are increasingly turning to "city agriculture" to produce cheap, locally grown fruit and vegetables.

Among skyscrapers and housing estates, previously vacant lots are being used to produce millions of tons of organically grown food that experts say are "greener" and cheaper than commercially grown produce.

But while many countries are in the early stages of their urban agriculture development, China, Japan and Cuba have had successful city farms for decades.

Cuba's model of environmentally friendly and sustainable urban agriculture has been an inspiration for numerous city projects around the world.

When the Soviet Union collapsed in 1991, Cuba's supplies of cheap oil suddenly dried up, plunging the country into a severe recession referred to as "the Special Period."

Farming in Cuba until then had relied heavily on oil to drive tractors and other heavy machinery, so there was a fundamental reorganization of food production, leading to a boom in urban organic agriculture.

Today, Cuba's capital Havana, which has a population of just over 2 million, has about 200 city farms that grow lettuce, tomatoes, peppers, sweet potatoes, spinach, herbs and other crops that are sold cheaply in local markets.

Wendy Emmett of the UK-based Cuba Organic Support Group, told CNN: "Cuba has been an inspiration, especially in the U.S. and the UK. They showed us what could be done when there is community will and a political will."

A similar community-based initiative has just been launched in Germany's financial capital Frankfurt. Groups can lease land from start-up company Meine Ernte, which provides tools and even sows the seeds, although the lease holders have to take care of the crops.

Frankfurt-based lawyer Mortem Simm said: "Most people just go to the supermarket and they can buy everything at any time of the year, but this brings us back to nature."

Meine Ernte already has six plots in German cities growing cucumbers, potatoes, carrots and tomatoes.

Natalie Kirchbaumer, co-founder of Meine Ernte, said: "They have to put in one or two hours per week, yes they have to work on the plants a little, but everything is there."

A slightly different model of urban farming is being deployed in parts of Africa, although it is still employing the same philosophy of community cooperation.

In the densely populated slum of Mathare in Nairobi, Kenya, an Italian charity Cooperazione Internazionale (COOPI) is helping communities to grow food in large compost bags, which are designed to provide the maximum output of produce in minimum space.

Its "farm-in-a-sack" project provides poor families with more than 40 seedlings, which can be grown into food in just a few weeks. Each "base" or mini-farm can provide vegetables such as spinach for 150 families, says COOPI.

Claudio Torres, from COOPI, said: "There are two effects. There is the main effect that they really have more food and that's like nutrition and micronutrients. But also, as you can see, this brings together the community."

A third of Africa's population already lives in urban areas, a figure that the World Health Organization expects to grow, so urban agriculture is increasingly seen as a back-up to commercial farming to meet the food requirements of millions of people.

And while North America may not have the food and water shortage problems of some African nations, urban farms are still expanding in major cities such as Vancouver on the west coast of Canada.

Michael Levenston, the executive director of City Farmer, part of Canada's Office of Urban Agriculture, told CNN that there were a number of models being deployed.

"There are people growing stuff in their back gardens and then there are bigger models like the University of British Columbia, which has a market-sized farm in the center of the city selling produce every Saturday at a farmer's market ... that is a very strong and vibrant entity," he said.

The United States has sizeable urban agriculture projects in Atlanta, Chicago, Detroit, Los Angeles, Minneapolis, New York and Pittsburgh. One U.S. collective of urban farmers says it is has 800 city-based plots that last year produced 150 tonnes of food.

The group Urban Farmers says on its MySpace page: "We locate and secure unused land, space, rooftops and walls for the purpose of bringing people together to plant organic food gardens in low-income urban areas throughout America and abroad."

In the UK, urban agriculture has not been adopted with the same gusto as other wealthy, densely populated countries such as Japan, although it is growing according to some experts.

Ken Elkes of the Federation of City Farms and Community Gardens, told CNN: "There has been an increase of 190 members in the last two years. But it takes a lot of time and a lot of effort to run and maintain a city farm."

And that's where community spirit and a cooperative effort, as embodied by the urban farmers in Cuba, come in.