Arriving at the fourth country on her Latin American tour, Rosalyn Carter told Peru, which has been increasing its weapons purchases, that there is a "need to control the growth of armaments throughout the world."

Mrs. Carter, who is touring seven Latin American countries as the President's representative, reminded Peruvian leaders that their government had signed an agreement with seven other Latin Americans countries in December 1974 to limit the acquisition of offensive weapons.

She noted that President Carter said April 14 that if the declaration "were implemented, it would not only benefit all of the people in this region, but it would be a shining example to the whole world."

Her blunt comments came in an airport speech after she arrived from Quito, where she had two days of discussions with leaders of the military junta that rules neighboring Ecuador, a traditional adversary of Peru.

Her message to both nations is that the United States wants friendly relations with each of them and hopes that they will move to reduce tension in the Andean region, according to U.S. officials traveling with her.

En route to Lima, Mrs. Carter played down the demonstration in Quito yesterday by about 200 young people who threw stones and bricks at reporters covering her trip and shouted "Rosalyn Carter, go home."

She said she did not know anything about the demonstration, which took place across the street from a building in which she was holding meetings, and that the demonstrators were more concerned about backing a teachers' strike than they were about her.

[In Washington, President Carter told a group of congressmen that he had talked to his wife by telephone and she had told him she didn't know anything about the demonstration. "The thing that made it a headline in The Washington Post was that a news reporter got hit on the ankle," he said.]

In Quito Mrs. Carter was told that Ecuadorian leaders were "terrified" over the Peruvian arms buildup, according to one Ecuadorian Cabinet minister who attended the meetings with her.

"They have sophisticated Soviet planes and tanks and we don't have anything like that," the official said.

Peru, which started buying tanks from the Soviet Union in 1973 and negotiated the purchase of Soviet fighter-bombers late last year, says it is merely trying to modernize its weapons and wants new arms only for defense.

Ecuador's leaders asked Mrs. Carter if the United States would reconsider its veto of Israel's proposed sale of 24 Kfir jets to Ecuador or if the United States would sell them sophisticated fighter planes.

As she said at a news conference in Quito yesterday, Mrs. Carter told them that her husband's veto in February of the Kfir sale was part of a global policy to "stop the armament escalation." The Kfir uses American made parts, which gives Washington veto power over its sale to third countries.

When the Ecuadorians pointed out that Peru has sophisticated weapons, she said she would "mention that to Jimmy" and apparently left open whether the United States would sell similar planes to Ecuador.