Late last spring, Portland newspaper readers found an advertisement by Mayor Neil Goldschmidt.

Sixty of their fellow citizens, the mayor announced in an open letters, had just complete a proposal to help ensure "an adequate, affordable" supply of energy for the future.

Goldschmidt's timing was fortunate. With turmoil in Iran and lines forming at gasoline stations in the United States, the climate was right for the results of a 3 1/2-year study called the Portland Energy Conservation Project.

Now, the city has adopted and is preparing to implement what its designers call the most sweeping energy conservation program in the nation.

Among other things, the city would give homeowners five years to "weatherize" their homes, making enough improvements to pass a city energy conservation audit. After that, if their homes are not rated "energy efficient" they would not be permitted to sell them.

Similarly, apartment owners who don't weatherize to meet city standards within five years would not be permitted to rent a vacated apartment to a new tenant. And businesses would not be allowed to sell or remodel buildings that did not meet the efficiency standard.

The program also would:

Revise zoning laws to promote higher population desity near the main public transit routes.

Authorize the city to hire an expert on solar energy and require builders to provide property owners with information on solar systems in new construction.

Place on the 1980 ballot a penny-a-gallon gasoline tax proposal to finance traffic flow improvements.

Require trash haulers to offer customers without charge the option of separating their recycable garbage. In turn, the haulers would defray their own costs by selling reclaimed materials.

Backers say the program would save up to 35 percent of the energy the city would otherwise be needing in 1995 -- or the equivalent of 13 million barrels of oil a year at current prices. they concede that it would initially cost property owners at least $300 million -- about $1,350 per home -- but contend that the owners would make back that much, or more, in heating and cooling cost savings in the long run.

"We don't want to drive people or businesses out of Portland," said city energy adviser Marion L. Hemphill. "The bottom line is cost effectiveness. eThe improvements will pay for themselves; it will be to your economic advantage -- your own self-interest -- to make these improvements."

Portland's conservation plan follows a series of measures over the years that have earned Oregon a reputation for no-nonsense innovation in protecting its environment and saving energy.

Nevertheless, the mandatory aspects of the Portland plan are meeting some strong resistance here. Opponents say the plan represents in unprecedented government intrusion on the rights of property owners.

"It's a good idea but the mandatory provisions are unconstitutional -- a violation of individual freedoms," said Multnomah County Commissioner Dan E. Nosee. "Five years from now -- that's 1984 -- we'll have Orwell's Big Brother right here, looking over the shoulders of homeowners and businesses, all in the name of the police powers of the state."

Nosee, an official of the county that includes the city of Portland, is organizing a petition drive to challenge the plan's mandatory provisions. He predicts that he will have little difficulty getting the 15,000 signatures needed to place a proposed city charter amendment on the fall 1980 ballot.

The city's plan dates to late 1973, when Oregon, heavily dependent on hydroelectric power, was hard hit by drought and the effects of the Arab oil embargo. The state mandated cutbacks in industrial power use, limitations on commerical lighting and other measures.

Later, Goldschmidt, who now is U.S. secretary of transportation, and other city officials realized that municipalities are particularly ill-equipped to handle energy problems. So they set into motion a study that led to an energy conservation plan.

During the past year, backers, "sold" the program in a flurry of public service announcements on radio and television and in the published open letter from the mayor, signed "Neil."

Finally, with energy experts predicting that Oregon will face sizable power deficits in the early 1980s, the proposal was submitted in August to the City Council.

One month later the council formally adopted the plan. It is expected to be operational by Jan. 1.