More than 5,000 people from around the world have gathered for a five-day meeting on the problems of a rapidly urbanizing planet.

The third United Nations World Urban Forum opened Monday in Vancouver, where the UN held its first global meeting on human settlements 30 years ago.

The main item on the agenda is how to make life better in the world's fast-spreading cities.

In a speech to the opening session, Prime Minister Stephen Harper called mass migrations from country to city an "irresistible phenomenon" and "perhaps the most important trend of our time."

The federal government, as host, is spending $30 million on the event.

Similar forums were held in Barcelona two years ago and Nairobi two years before that, bringing together poor people and politicians, academics and activists.

"We hold a very unique position in the meeting business, you could say, in the sense we have anything from slum dwellers to prime ministers and ministers, and they all meet on an equal base," said the UN's global co-ordinator for the forum, Lars Reutersward.

Reutersward said $30 million wouldn't buy a new sewage treatment plant or many kilometres of new roads, but it is buying inspiration for those who attend, particularly from the developing world.

About 5,500 delegates had picked up their credentials by Monday afternoon. Organizers expected the total to reach 8,000 to 9,000.