FIANARANTSOA, Madagascar (AP) _ Pope John Paul II celebrated an open air Mass in an impoverished farming region today and said that despite tough economic times the faithful should not use artificial birth control.

''While it appears necessary to be aware of family planning, couples... should use natural methods,'' the pope told the 100,000 people who gathered for the Mass.

The Roman Catholic Church approves only what it calls natural family planning, or abstinence during a woman's fertile period.

Many Western nations have pressured Third World countries to control their population explosions, which hinder economic growth and are a chief factor in environmental degradation.

The pope said that although the church's teachings seem difficult, natural methods will work and the faithful should reject ''contraceptive imperialism.''

John Paul arrived 90 minutes late for his last Mass on this Indian Ocean island. Fog had closed the airport 240 miles south of the capital of Antananarivo.

After the Mass, the pope flew back to the capital and boarded an Air Madagascar Boeing 737 for the hour's flight to the French island of Reunion, the second of four stops on his fifth African tour. On Tuesday he travels to Zambia then Malawi.

President Didier Ratsiraka called the pope's visit ''a source of comfort for those who are suffering'' in this impoverished country.

In Fianarantsoa, the pope blessed 100 lepers who kneeled in a sandy field near the airport before saying Mass from a raised, wooden altar covered by a canopy.

The pope today spoke of the serious environmental problems facing Madagascar and other developing countries and reaffirmed his call Sunday for an international effort to safeguard the environment.

''From the north to the south of your great island, I have been able to admire the beauty, diverse richness of the Earth and its fruit. And yet the way it is being used risks its degradation and sterilization.''

Saving this 225,000-square-mile island that split from the African mainland more than 150 million years ago has become a priority of many international environmental groups.

Ninety percent of the island's plants and animals are found nowhere else, including hundreds of species of orchids and 29 species of lemurs, small primates whose name derives from the Latin word for ghost because of their nocturnal habits.

The destructive farming methods of the island's population, constantly seeking fresh terrain to plant rice and other crops, has caused widespread deforestation.

The Mozambique channel, off the island's west coast, is so red from the infusion of soil washed in by erosion that it is clearly recognizable to astronauts circling the globe.

On Sunday, the pope spoke on the environmental threat to a gathering of foreign diplomats. He said he supports international efforts to halt the dumping of toxic wastes in poor countries and development that endangers the world's remaining rain forests.

Also Sunday, the pontiff beatified a Malagasy woman credited with keeping the faith alive 100 years ago after the queen had banned all Roman Catholic services. Beatification is a first step toward possible sainthood.

The church has been openly critical of Madagascar's government. Since a communist revolution in 1975, the country's economy has declined and recently, the government made policy alterations to assure foreign aid.