Pope John Paul II celebrated an open air Mass in an impoverished farming region Monday 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 celebrating Mass from a raised, wooden altar covered by a canopy.