Thousands of people many dressed in white and carrying the purple, gold and white banners of the suffragettes, are expected to march along Constitution Avenue Sunday and rally at the Capitol to urge Congress to rescue the Equal Rights Amendment to the Consitution.

Officials of the National Organization for Women, which is coordinating the demonstration, estimated yesterday that at least 30,000 people from across the country will participate. More than 325 groups are sponsoring the demonstration, according to NOW president Eleanor Smeal.

"Religious groups, labor unions, civil rights organizations, educational groups - they are all coming to Congress to demonstrate the vitality of ERA," she told a news conference yesterday.

The Equal Rights Amendment for women has been ratified by 35 state legislatures, three short of the number that must approve the measure before it becomes a Constitutional amendment. The deadline approval by 38 states is March 22, 1979 and ERA supporters have said they do not believe there is enough time to get ratification in three additional states.

Legislation to give the ERA another seven years for ratification is expected to be voted on soon by the House Judiciary Committee and by the full House later this month. ERA supporters plans to stay over after the march and lobby Congress on Monday for the ERA extension resolution.

Sunday's demonstration will begin about 11 a.m. at the staging site at the 14th Street end of the Mall. Delegations led by a Girl Scout honor guard will begin marching along the Constitution Ave. route to the Capitol about noon, according to march planners.

Speakers scheduled to address the rally include Eleanor Holmes Norton, chairwoman of the Equal Employment Opportunity Commission, Reps. Margaret Heckler (R-Mass.). Elizabeth Holtzman (D-N.Y.), and Barbara Mikulski (D-Md.); actresses Mario Thomas and Jean Stapleton. Dorothy Height of the National Council of Negro Women. Ronald Brown of the National Urban League, actor Elliot Gould, Betty Friedan. Gloria Steinem. Bella Abzug. Millie Jeffrey of the National Women's Political Caucus, and NOW's Smeal.

Organizers say they expect the rally to end about 4 p.m.