Events have been taking place in more than 100 countries to mark World Environment Day, which is promoting the idea of "green cities".
Pedal power ruled at an environment rally in Taiwan
The UN estimates that more than 60% of people will live in cities by 2030.
The day highlights ways of managing expansion to benefit people and their surroundings. Events included rallies, tree-plantings and clean-ups.
The UN environment agency says local governments, businesses and industry must work together to build change.
Waste and pollution
Almost half the world lives in cities and a third of these people dwell in slums and squatter settlements, mainly in the developing world.
Air pollution is a major problem.
Half a million deaths are caused by particulates and sulphur dioxide, which comes largely from car fumes.
Four-fifths of the gas linked to global warming comes from cities, the United Nations Environment Programme (Unep) says.
This year's World Environment Day has focused on ways of making cities more environmentally friendly and resource-efficient.
Events held across the world included a ban on cars in the Greek port of Zakynthos, tree-planting along the tsunami-devastated coastline of Sri Lanka, and a fashion show in Japan encouraging workers to abandon ties and suit jackets, in order to cut air conditioning costs in summer.
In San Francisco, mayors from more than 50 cities including Shanghai, Kabul, Sydney and Rome plan to sign up to a scheme setting new green standards for city planning.
"We need to get cities right, because they suck in resources and they pump out waste," Unep's Nick Nuttall said.
World Environment Day was established by the UN General Assembly in 1972 to mark the opening of the Stockholm Conference on the Human Environment. It is celebrated each year on 5 June.