THE world's major cities are sending an advance message to the United Nations Conference on Environment and Development (UNCED), scheduled to be held in Rio de Janeiro next June. The message is that the conference needs their input and cooperation to succeed and that they are ready to give it.Leading officials of 22 major cities - from Beijing to New Delhi and Mexico City to New York - are expected to sign a declaration to that effect this afternoon here at a Montreal hotel. They have been attending the Third Summit of the World's Major Cities, an idea launched by Tokyo's top official in 1985. This is the first meeting to focus on one topic: the environment. Seated along two sides of a long table appropriately covered by a green table cloth, the city leaders commiserated over many of their common problems - from traffic to garbage - and agreed that these are at the root of many of the world's worst pollution problems. That fact makes their response all the more important to the rest of the world. Few leaders present had any quarrel with UNCED Secretary-General Maurice Strong's assertion at the conference that "the battle for the survival of the planet will b e won or lost in the cities." By the year 2000 more than 50 percent of the world's population will be city dwellers. Most will live in the developing world. Though the cities represented at the summit are at vastly different stages of development, they agreed that development strategy in all cities must change. Mayor Jean Dore of Montreal, chairman of the summit, said environment must be at the heart of the new approach. "It is urgent for us ... to put an end to the daily aggression on the environment," he said. UNCED chief Strong said that lifestyles of the well-to-do must also change. Existing patterns of production and consumption of resources, he said, have put the whole human community "at risk." Noting that many cities are taking strong steps to restrict private car use, he said he would not be surprised if small vehicles someday are made available on a pay-as-you-use basis in congested urban areas. Tokyo Gov. (mayor) Shunichi Suzuki, particularly commended by Strong for his vigorous efforts against air and water pollution in recent years, spoke of his city's promotion of recycling and use of waste to produce heat and electricity. Tokyo also wants to make use of waste heat from its subway system. "The time has come to think of city management globally, to pool our wisdom," said Governor Suzuki. One of the great problems for many cities in developing nations is the speed of growth. New Delhi Chief Secretary Rajinder Kumar Takkar calls it the No. 1 problem. "We are growing too fast for the city managers," he says. "We're not able to give all the housing, the electricity, and the roads that are required." Still, he adds: "One need not feel guilty about growing urbanization.... It's gone hand in hand over the years with civilization. What's important is how you manage it." Many leaders stressed how much their cities were already doing. Beijing Vice Mayor Lu Yucheng spoke of his city's intensive effort to add more parks and trees and said last year there were 39 fewer "dust days" than the year before. He said that more than 4 million trees have been planted along five major highways in the Beijing area. Cairo Gov. M. Omar Abdel Akkar, who noted that in some parts of his city there are as many as 100,000 people per kilometer, said Cairo is working hard to increase supplies of pure water and electricity and to develop and expand its subway system. Similarly, the governor of Jakarta, Wiyogo Atmodarminto, says his city hopes to supply 75 percent of its population with pure water from the city system by 1995 and is now developing a transfer station where garbage that is normally shipped to landfills outside the city can be sent first for recycling and compressing. "But Jakarta is already getting cleaner - I think it's cleaner than Amsterdam," says Governor Atmodarminto. He says his city has benefited particularly from sister city arrangements with Ro tterdam, Tokyo, Seoul, and Los Angeles. Much has been said at the conference of the North-South divide and the need for equity in all future development. "Development of our cities must be based on ... equal opportunity for all citizens," says Mayor M. Ernest N'Kuomo Mobio of Abidjan, Ivory Coast, articulating the views of his summit colleagues. "The North has to agree to sharing part of the planet's wealth with the other citizens of the planet." Mr. N'Kuomo then cited an example of such sharing, noting that Abidjan has a particularly high unemployment rate among youth. He said that Montreal and the Canadian government have helped his city begin a project to increase poultry breeding as a source of jobs. It has been very successful, he says, in not only creating several hundred new jobs but in increasing the food supply.