By 2030, more than 60% of the world's population will live in cities, up from almost half now and just a third in 1950, UN Secretary-General Kofi Annan said. The growth poses huge problems, ranging from clean water supplies to trash collection.

This year's theme for World Environment Day is better ‘green’ planning for the world's burgeoning cities, many of them blighted by air pollution, fouled rivers and poor sanitation.

"Already, one of every three urban dwellers lives in a slum," Mr Annan said in a statement. "Let us create green cities," he said, adding that unless planning improved, the UN goal of halving poverty by 2015 would not be met.

Activists around the world mark June 5 as the UN's World Environment Day.

In San Francisco, the main host of the event, mayors from more than 50 cities including Shanghai, Kabul, Buenos Aires, Sydney, Phnom Penh, Jakarta, Rome and Istanbul planned to sign up for a scheme setting new green standards for city planning.

In Sri Lanka, a group was to plant trees to help build up the coastline after the devastating December 26 tsunami, while in Greece, the port of Zakynthos planned to ban cars and allow free public transport.

Around Australia, green groups and local councils organised festivals to promote awareness of environmental issues from recycling to tree planting to cleaning up waterways.