Peace groups around the nation are preparing vigils and rallies on Sunday to demand legal and diplomatic alternatives to war.

The American Friends Service Committee, Pax Christi USA, the War Resisters League and others have asked their members to participate in the local gatherings, including a march and rally in New York, beginning at Union Square.

''We're encouraging peace groups throughout the country to get together and formulate their own way to speak out and promote peace,'' said Janis Shields, spokeswoman for the American Friends, a Quaker organization.

The idea, Ms. Shields said, is ''to call attention to the fact that not everyone is calling for war in these trying days.''